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Infrared spectroscopy of carbocations in helium droplets
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Infrared spectroscopy of carbocations in helium droplets
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Content
INFRARED SPECTROSCOPY OF CARBOCATIONS IN HELIUM DROPLETS
by
Swetha Erukala
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(PHYSICAL CHEMISTRY)
August 2022
©Copyright Swetha Erukala
ii
Acknowledgements
I thank my parents and sister for their boundless love and constant support. My mother is
a strong-willed woman, she dreamt that her daughters should have the highest level of education
and be independent and I am extremely happy that I made her dream come true.
I thank my advisor Prof. Andrey Vilesov for his constant guidance and support throughout
the process of my PhD. I have learnt to be a good experimentalist from him and a major part of
my growth process as a researcher is because of him. His enthusiasm and love for science is
contagious and I too have been infected at times by it. He is a person I always looked up to,
admired and loved and I am happy to have known him. I hope I took some of his good qualities in
these five years. In addition to scientific support, he has always given me some great advice on
life which I will continue to incorporate.
I thank Dean Guyer of Laser Vision for his support when I was troubleshooting laser. I
learnt a lot from him and have been very much moved by the immense help he provided through
phone calls and emails. I also want to thank the technical help I got from engineers at Coherent,
National Instruments and Extrel especially during the time of pandemic when I was working alone
in the laboratory. I thank Prof. Jahan Dawlaty and group, Corey Schultz for constantly checking
up on me in laboratory during the pandemic. I also like to thank my committee members: Prof.
Vitaly Kresin and Prof. Oleg Prezhdo. I thank Prof. Surya Prakash for his inputs on the systems
that could be studied with our setup.
In the initial years of my PhD, it was a challenge to adapt to US and make friends. I am
very thankful to Sourav Dey, Saikiran Kotaru and Anwesha Maitra who have become my family
iii
here in US and on whose love and support I constantly depended on throughout my stay here.
Together with Pratyusha Das, Laura Mugica, Anju Nalikezathu, Sraddha Agrawal, Anuj Pennathur
and Kethika Garg I had some of the most happy and wonderful memories. Towards the end of my
stay in LA I also managed to find a movie partner in Mythreyi Rayaluru. It is a blessing to have
friends whom you can ring up at midnight if any problem comes up and I have that privilege with
Vaddi Yaswant, Jigyasa Gaurav, Thyagarajulu Gollapalli, Kalyan and Saiphaneendra Bachu.
Alexandra Feinberg and I joined the group in the same year and it was nice to have a friend
inside the group who was going through similar experiences at the same time and I thank allie for
being a friend to me. For the most part of my PhD, I felt I worked alone, but with Dr. Cheol Joo
Moon I had the experience of working as a team and I thoroughly enjoyed doing experiments with
him and I am thankful to him for that experience. Dr. Deepak Verma laid the groundwork for my
PhD by building the robust machine I did my experiments on. I would like to thank the other
members of the Vilesov group: Amandeep Singh, Dr. Sean O’ Connell and Dr. Rico Mayro
Tanyag. I also thank Michele Dea, to whose office I constantly ran and bugged to place the orders
for our lab. I thank Magnolia Benitez and Jaime Avila for their assistance in non-scientific matters.
I am thankful to the process of PhD itself; it has made me a confident person and instilled
self-belief in me.
The work is this thesis was supported by NSF under Grant Nos. CHE-1664990, CHE-
2102318 and DMR – 1701077.
iv
Table of Contents
Acknowledgements.................................................................................................. ii
List of Tables ........................................................................................................ vii
List of Figures ...................................................................................................... viii
Abstract .................................................................................................................. xii
Chapter 1. Introduction .......................................................................................... 1
1.1 Scope of Work .............................................................................................. 2
1.2 List of publications ........................................................................................ 4
Chapter 2. Infrared Spectroscopy of Water and Zundel cations in Helium
Nanodroplets ......................................................................................... 5
2.1 Introduction ..................................................................................................... 5
2.1.1 Vibrational spectroscopy of molecular ions .............................................. 5
2.1.1a High resolution spectroscopy of ions in a discharge .......................... 6
2.1.1b Matrix Isolation Spectroscopy ........................................................... 7
2.1.1c Infrared Laser Photodissociation ........................................................ 8
2.1.1d Vibrational Spectroscopy in cryogenic ion traps ............................... 9
2.1.2 Vibrational spectroscopy of ions in Helium Droplets ............................. 10
2.2 Experimental Setup ....................................................................................... 12
2.3 Results and Discussion ................................................................................. 15
2.3.1 Water Ions .......................................................................................... 15
2.3.2 Zundel Ions ......................................................................................... 18
2.3.3 Formation of the Doped Ions and Laser Induced Release of Free
Ions ..................................................................................................... 21
2.4 Conclusions ................................................................................................... 26
2.5 References ..................................................................................................... 27
v
Chapter 3. Rotation of CH3
+
cations in Helium Droplets ................................... 33
3.1 Introduction ................................................................................................... 33
3.2 Experimental Parameters .............................................................................. 35
3.3 Results and Discussion ................................................................................. 36
3.4 Conclusion .................................................................................................... 42
3.5 References ..................................................................................................... 44
Chapter 4. Infrared Spectroscopy of Carbo-Cations upon Electron
Ionization of Ethylene in Helium Nanodroplets ............................ 47
4.1 Introduction ................................................................................................... 48
4.2 Experimental ................................................................................................. 50
4.3 Results ........................................................................................................... 52
4.4 Discussion ..................................................................................................... 61
4.4.1 Formation of ions by charge transfer from He
+
....................................... 61
4.4.2 Formation of larger ions by ion molecule reactions ................................ 64
4.4.3 Free Ions ................................................................................................... 66
4.4.4 Spectroscopy and comparison with gas, matrix and tagging ................... 67
4.4.5 Rotation of ions in helium droplets ....................................................... 69
4.2 Conclusion .................................................................................................... 72
4.3 References ..................................................................................................... 73
Chapter 5. Infrared Spectroscopy of Ions and Ionic Clusters upon
Ionization of Ethane in Helium Droplets .......................................... 81
5.1 Introduction .................................................................................................. 81
5.2 Experimental Setup ...................................................................................... 82
5.3 Results .......................................................................................................... 84
5.4 Discussion .................................................................................................... 91
5.5 Conclusions .................................................................................................. 96
5.6 References .................................................................................................... 96
vi
S. Supplementary Information ........................................................................... 101
Chapter 6. Infrared spectroscopy of carbocations in Helium droplets .......... 104
6.1 Introduction ................................................................................................ 104
6.2 Results and Discussion .............................................................................. 105
6.2.1 C2H
+
....................................................................................................... 105
6.2.2 C3H2
+
...................................................................................................... 107
6.2.3 C3H3
+
...................................................................................................... 110
6.2.4 C3H4
+
...................................................................................................... 113
6.2.5 C3H5
+
...................................................................................................... 115
6.2.6 C3H6
+
...................................................................................................... 117
6.2.7 C4H2
+
...................................................................................................... 119
6.2.8 C4H3
+
and C4H4
+
.................................................................................... 121
6.2.9 C4H5
+
...................................................................................................... 123
6.2.10 C4H7
+
................................................................................................... 125
6.3 Conclusion ................................................................................................. 127
6.4 References .................................................................................................. 127
Chapter 7. Conclusion and future outlooks....................................................... 137
vii
List of Tables
Table 3.1. Molecular constants of the v3 vibrational state of CH3
+
gas phase, in He droplets and
in CH3
+
-He2 complexes. All values are in units of cm
-1
. ...............................................................40
Table 4.1. C-H vibrational frequencies for cations obtained in gas, via tagging, in solid matrix and
in helium droplets. The last column given the frequencies for the corresponding neutral species.
All values and in units of wavenumbers. .......................................................................................68
Table 6.1 C-H vibrational frequencies of C3H2
+
obtained in helium droplets and measured with
Ne tagging. Both measurements were done with allene precursor. The last two columns show the
results of quantum calculations using coupled cluster methods. All frequencies are in cm
-1
.....108
Table 6.2. Comparison of C-H vibrational frequencies for the two isomers of C3H3
+
obtained in
helium droplets with different precursors, with Ar tagging measurements and anharmonic
calculations using second order perturbation theory. All frequencies are in cm
-1
.......................110
Table 6.3. C-H vibrational frequencies for the two isomers of C3H4
+
obtained in helium droplets
with different precursors compared with the frequencies obtain in Ar Matrix and results of
harmonic vibrational calculations using B3PW91/aug-cc-pVTZ. All frequencies are in cm
-1
...113
Table 6.4. C-H vibrational frequencies for the two isomers of C 3H 5
+
obtained in helium droplets with
different precursors, with Ar tagging and anharmonic calculations using second order perturbation theory.
All frequencies are in cm
-1
. ..............................................................................................................117
viii
List of Figures
Figure 2.1 Water cooled multiple inlet-outlet discharge cell used to generate ions for gas phase
ion spectroscopy
1
.............................................................................................................................6
Figure 2.2. Sampling configuration used in matrix isolation methods, excited neon in discharge
ionizes molecule XY and the ions are deposited onto cryogenic surface.
8
.....................................7
Figure 2.3. a) Schematic of IRPD set up consisting of ion discharge source skimmed and mass
selected with tag atoms prior to laser interaction in octupole ion trap
17
b) the tag dissociates upon
vibrational excitation and the photofragments are mass selected to record the spectra ..................8
Figure 2.4. Schematic of infrared multiphoton dissociation ion spectroscopy using cryogenically
cooled 22 pole ion trap to obtain ions at ~5 K.
18
.............................................................................9
Figure 2.5. Schematic of helium droplet setup for molecular ion spectroscopy. He droplets are
produced from a cold pulsed nozzle in the source chamber. The He droplets capture dopants in the
pickup chamber and traverse into the detection chamber equipped with two electron impact
ionizers: External and Probe. The droplets are ionized in the external ionizer and interact with the
counter propagating infrared laser beam once in the ion region of the probe ionizer. The released
ions are mass selected by the quadrupole mass spectrometer. .......................................................12
Figure 2.6. (a) Spectrum of H
2
O
+
in helium droplet of ~ 7×10
3
atoms as measured at laser pulse
energy of ~ 5 mJ/pulse. Measurement time is about 10 min. The origins of the ν1 and ν3 bands of
the free ions are marked by green vertical bars, whose heights are proportional to the infrared
intensity.
19
(b) Intensity dependence of v3 band versus the laser pulse energy (E): Black squares
represent the experimental results while the red curve shows a fit by I = a∙E
3.9
for E < 2 mJ. .....15
Figure 2.7. Spectrum of H5O2
+
in He droplets (red), compared with the spectra of He-H5O2
+
(blue)
27
and Ar-H5O2
+
(black).
28
The band origins of the free ions are marked by green vertical bars,
29
whose heights are proportional to the infrared intensity ................................................................18
Figure 2.8. Efficiency of the ion production, A, vs reduced laser flux, F. See text for more details.
Red, blue and green curves are for average droplet sizes of 7000, 2000 and 800 atoms, respectively.
........................................................................................................................................................25
Figure 3.1. (a) Stick spectrum of the v3 band of free CH3
+
with transitions from K" = 0, 1 based
on spectroscopic constants in Ref.
15
. The spectral lines are denoted by (J', K') (J", K"). The
heights of the bars give the calculated relative intensity of the lines assuming 1:1 population ratio
of the K"= 0 and 1 states. (b) Spectrum of the v3 band of CH3
+
in helium droplets. The transitions
with unresolved J- structure are denoted by K' K". (c) Simulated spectrum of CH3
+
in helium
with modified rotational constants as described in the text. .........................................................36
Figure 3.2. CH3
+
signal intensity vs pickup pressure of CH4 recorded with laser frequency tuned
to the maximum of the v3 band of CH3
+
in He droplets at 3131.6 cm
-1
. The red curve represents the
fit of the data points by Poisson probability for capture of a single molecule per droplet. ...........37
ix
Figure 4.1. Mass spectra obtained with the probe ionizer. Black trace - baseline due to residual
gas in the detection chamber. Red trace - neat helium droplets. Blue trace - helium droplets dopped
at 4.5×10
-6
mbar of ethylene. The traces are plotted to the same scale. .......................................52
Figure 4.2. Ethylene pickup pressure dependence of the signal at M=26, M=27 and M=28 is shown
by black squares. In c) the baseline signal due to He7
+
and N2
+
at zero ethylene pressure was
subtracted. Red curves are fits with Poison dependence for pickup of single molecule ...............53
Figure 4.3. The total yield of ions upon laser irradiation of the ethylene doped ionized droplets.
The insert shows an additional scan in the indicated range recorded with about a factor of ~10
higher amplification. The assignment of the spectral peaks is indicated .......................................54
Figure 4.4. Mass spectra measured with laser parked at the spectral peaks as indicated in the
legend. Trace a) was obtained with laser blocked .........................................................................55
Figure 4.5. Spectra recorded at M=14 (pink), 26 (blue), 27 (red) and 28 (black) ........................57
Figure 4.6. Laser pulse energy dependence of the signal for C2H2
+
at 3140.0 cm
-1
(blue), C2H3
+
at
3145.7 cm
-1
(red) and C2H4
+
at 3105.3 cm
-1
(black) ......................................................................58
Figure 4.7. Ethylene pickup pressure dependences of the laser induced signal for C2H2
+
at 3139.8
cm
-1
, C2H3
+
at 3144.4 cm
-1
,and C2H4
+
at 3105.3 cm
-1
as measured at M = 26 (a), M = 27 (b), and
M = 28 (c), respectively. Curves are fits of the data points by Poisson probability for the capture
of k = 1 (red) and k = 2 (blue) molecules per droplet. ...................................................................59
Figure 4.8. The comparison of the spectrum of C2H3
+
-Ar with the spectrum of C2H3
+
in helium
droplets ...........................................................................................................................................69
Figure 5.1. The mass spectrum obtained in probe ionizer of QMS upon doping He droplets with
ethane at 4×10
-6
mbar ethane. The intensity of the most intense peaks around 6000 is incorrect due
to saturation of the detection system. .............................................................................................84
Figure 5.2. Total ion yield upon laser irradiation of helium droplets doped at 4×10
-6
(black trace)
and 2×10
-5
mbar (red trace) ethane pickup pressure. .....................................................................85
Figure 5.3. Mass spectra recorded upon laser excitation at a) 2987.19 cm
-1
and b) 2811.3 cm
-1
respectively. Ethane pickup pressure 2×10
-5
mbar. .......................................................................87
Figure 5.4. Infrared spectra recorded upon detection of mass 30 (red trace) and 60 (Black trace)
collected at Ethane pickup pressure 2×10
-5
mbar...........................................................................88
Figure 5.5. Pickup pressure dependences for laser induced signal as measured at indicated masses.
a) and b) were obtained with the laser tuned to the maximum of the v7 band of ethane clusters at
2987 cm
-1
and c) at 2811.3 cm
-1
. Curves are fits of the data points to Poisson probability of
capturing k molecules per droplet. .................................................................................................89
x
Figure 5.6. Infrared spectra of the v7 band of C2H6 in ionic clusters as recorded at of a) M=90, 89,
88 and b) M=60, 59 and 58. The frequencies of the v7 band in neutral ethane clusters in helium
droplets and in free molecules are shown by green and black sticks, respectively. ......................90
Figure S1. Ethylene pickup pressure dependence of the signal for C2H3
+
, C2H4
+
, C2H5
+
, and C2H6
+
as measured at M = 27 (a), M = 28 (b), M = 29 (c), and M = 30 (d), respectively. The droplets were
ionized by electron impact in the probe. In b) the baseline signal due to He7
+
and N2
+
at zero
ethylene pressure was subtracted. The red curves are fits to the Poisson dependence for the pickup
of single molecules. ....................................................................................................................101
Figure S2. The spectra obtained upon laser irradiation of the ethane doped ionized droplets.
Spectra recorded at M = 30 (black), 58 (red), 59 (blue), 60 (magenta), 88 (dark yellow), 89 (olive),
and 90 (orange). ...........................................................................................................................102
Figure S3. The comparison of spectra recorded at M = 29 upon laser irradiation of helium droplets
doped at 4×10
-6
(black trace) and 8×10
-6
mbar (red trace) ethane pick up pressure. The inset shows
an pick up pressure dependence of the laser induced signal for C2H5
+
at 3150.3 cm
-1
as measured
at M = 29. Curves are fits of the data points by Poisson probability for the capture of k = 1 (red),
k = 2 (blue), and k = 3 (magenta) molecules per droplet. ............................................................103
Figure 6.1. a) Infrared spectra of
C2H
+
obtained upon ionization of the acetylene doped helium
droplets upon detection of M=25. b) Mass spectra obtained with laser set at the maxima of three
spectral peaks in a) at 3182.5(black), 3145.1 (red) and 3111.4 cm
-1
(blue) frequency ...............106
Figure 6.2. a) Infrared spectrum of C3H2
+
obtained upon electron impact ionization of allene in
helium droplets b) Cyclic C3H2
+
c) Linear C3H2
+
. ......................................................................107
Figure 6.3. a) Infrared spectra of C3H3
+
obtained upon ionization of helium droplets doped with
propene (black trace), ethylene (red trace) and allene (blue trace) b) cyclopropenyl cation c)
linear propargyl cation ................................................................................................................110
Figure 6.4. a) Infrared spectra of C3H4
+
in He droplets obtained with propene (black) and allene
(red) precursors b) Allene c) Propyne ions. . ...............................................................................113
Figure 6.5. a) IR spectrum of C3H5
+
in helium droplets obtained using different precursors:
propene (blue trace), ethylene (black trace), n-butane (pink trace) and allene (red trace) b) allyl
cation c) 2-propenyl cation. .........................................................................................................115
Figure 6.6 Infrared spectra of C3H6
+
obtained in He droplets with propene precursors. ...........118
Figure
6.7. a) Infrared spectrum of C4H2
+
obtained upon ionization of acetylene doped helium
droplets b-e) Lowest energy isomers of C4H2
+
............................................................................119
Figure
6.8. a) Infrared spectra of C4H3
+
and C4H3
+
obtained upon ionization of acetylene doped
helium droplets b-d) Lowest energy isomers of C4H3
+
e-h) Lowest energy isomers of C4H4
+
...121
xi
Figure
6.9 a) Infrared spectra of C4H5
+
obtained upon ionization of in acetylene and ethylene
doped helium droplets b-e) Lowest energy isomers of C4H5
+
....................................................124
Figure
6.10.a) Infrared spectra of C4H7
+
upon ionization of helium droplets doped with acetylene
and transbutene. b-e) Lowest energy isomers of C4H7
+
..............................................................125
xii
Abstract
Carbocations play an important role as reactive intermediates in both laboratory and
astrophysical environments. Understanding their structure and stability allows one to better
estimate favorable reaction pathways and design chemical reactions. In addition to NMR,
vibrational infrared (IR) spectroscopy can also provide information on the structure of ions.
However, in comparison to neutral molecular species, IR spectroscopy of ions is much less
developed primarily to their high reactivity. This thesis presents the experiments on the IR
spectroscopy of carbocations in superfluid helium droplets which employ electron impact (EI)
ionization for the preparation of the ions. Due to the low temperature (~0.4 K) and homogeneous
superfluid environment of the embedded species the droplets present an ultimate matrix for
spectroscopic measurements. Because the ionization potential of He of ~24 eV is way above that
for any other species, any molecular ions can be produced and studied in the droplets. Here we
show that the EI of the droplets containing neutral precursor molecules yields ionic species
embedded in the droplets of few thousands of atoms, making it a convenient nanomatrix for
production and spectroscopic interrogation of a wide variety of embedded ions and ionic clusters.
Infrared spectra are obtained using release of the cations from the droplets upon laser excitation.
The topics of interest in this thesis include - cation-He interaction, ion – molecule and clustering
reactions at ultracold temperatures. Rotational resolved spectrum of CH3
+
ions was used to study
the interaction of a light rotor ion with helium environment. Previously it was assumed that He
around ions form a rigid "snowball" structure. We concluded that the effective rotor in helium has
two He atoms attached on the figure axis of CH3
+
with other He atoms in the first shell remain
fluxional. Besides, we report IR spectra of ions such as C2H2
+
, C2H3
+
, C2H4
+
, C3H3
+
, C4H5
+
,C4H7
+
etc., resulting from ionization, fragmentation and ion-molecule reactions of unsaturated ethylene
xiii
doped helium droplets upon electron impact ionization. Interestingly for saturated ethane
clustering was found to be favored over ion-molecule reactions at the low temperature in helium
droplet. In addition, we extend our experiments to various isomers of ions resulting from electron
impact ionization of droplets doped with various precursor molecules such as acetylene,
transbutene and propene. Many of the obtained IR spectra such as for C3H6
+
, C4H3
+
, C4H4
+
, C4H5
+
,
C4H7
+
, C2H
+
are reported for the first time in this thesis. The spectra will help understanding the
structure of these reactive carbocations.
1
Chapter 1. Introduction
1.1 Scope of work
The focus of this thesis is the infrared (IR) spectroscopy of carbocations in superfluid
helium droplets. In this approach the droplets are first doped with single or multiple precursor
molecules. The doped droplets are then ionized via electron impact (EI), the process which was
found to produce diverse ions resulting from dissociative ionization of the precursor molecules
and their ensuring ion-molecule reactions. The IR spectra of the ions inside the droplets are then
obtained by recording the signal from bare mass resolved ions released from the droplets upon
resonant laser excitation. IR spectra of a wide range of carbocations resulting from fragmentation
of the precursors and ion – molecule reactions were obtained using methane, acetylene, ethylene,
ethane, propylene, allene and transbutene precursors. In case when infrared spectra of the ions
were previously measured, our measurements give a good agreement of the vibrational frequencies
in the C-H stretching range. In addition, we report infrared spectra of larger ions, such as C3H6
+
,
C4H3
+
, C4H4
+
, C4H5
+
, C4H7
+
, which were never observed before via infrared spectroscopy. This
work shows the utility of IR spectroscopy in He droplets for study of diverse larger carbo-ions and
other molecular ions which may be particularly useful for determination of their structure and
possible isomer structures.
Chapter 2 describes the experimental set up and working principle of the ion spectroscopy
in helium droplets used in studying cations in this thesis. It starts with a brief review of the previous
spectroscopic experiments with ions in the gas phase and in molecular beams. The reactivity and
the space charge effects limiting the ion density in gas phase makes ions a difficult system to study.
Helium droplets appears to be an ideal matrix to study ions due to ultracold environment and weak
2
interaction of ions with He atoms. The experimental set up used electron impact ionization for the
first time to generate ions in droplets and acquired the spectra by recording the intensity of ions
released following absorption of multiple IR photons. To validate our technique, we measured the
spectrum of Zundel (H5O2
+
) ions which have been extensively studied before by tagging with
argon atoms. The spectra reveal two sharp infrared bands indicating a symmetric structure of
H5O2
+
which remains intact inside the homogenous environment of the droplets. The vibrational
bands in He appear to be a factor of 10 narrower as in the tagging experiments, an effect assigned
to low T = 0.4 K in the droplets. Model calculations are presented to explain the mechanism of ion
release from the droplets upon laser irradiation and the non-linear dependence of the IR signal on
laser flux. In addition, the spectrum of H2O
+
in helium droplets is discussed. This work is published
in Ref.4.
Chapter 3 describes the rotation of CH3
+
cation and its interaction with the surrounding He
environment. Analysis of the rovibrational spectrum of the v3 C-H stretch of CH3
+
revealed CH3
+
to be a prolate top in helium droplets with at least two He atoms attached perpendicular to the
plane of CH3
+
. The other atoms in the first solvation shell of CH3
+
likely exchange with more
distant He atoms of the superfluid droplet. This observation disputed the proposed rigid shell of
He atoms – Atkins snowball, around the cation. This work reports the first rotationally resolved
IR spectrum of a cation in He droplets. This work is published in Ref.3.
Chapter 4 shows that a variety of CXHY
+
cations in helium droplets could be produced by
electron impact ionization of ethylene doped helium droplets. The IR spectra of the primary ions
such as C2H2
+
, C2H3
+
, C2H4
+
and CH2
+
generated via charge transfer from He
+
ions to ethylene
molecules, dimers or clusters are presented and discussed. The frequencies obtained in the C-H
3
stretching range are well resolved (1-3 cm
-1
) and the shifts induced due to surrounding helium
environment are within ~ 20 cm
-1
allowing direct comparison from quantum mechanical
calculations to determine the structure. Carbocations containing three to four carbon atoms such
as C3H3
+
, C3H5
+
, C4H5
+
, C4H7
+
resulting from the reaction of primary ions with the ethylene
molecule are identified. These ion-molecule reactions taking place in the ultracold matrix can serve
as the laboratory simulations of the reactions occurring in interstellar space. The large range of
carbocations obtained with this single precursor shows the versatility of our technique of electron
impact ionization in generating different ions from different precursors, which range could be
extended upon application of different precursor molecules. This work is published in Ref.2.
Chapter 5 extends our study to ionic clusters of ethane in helium droplets. Unlike for
unsaturated ethylene, higher chain carbocations resulting from ion – molecule reactions were not
found. Instead, ion-induced dipole attractions resulting in ionic clusters seem to be dominant for
saturated ethane. Larger ethane clusters with ion cores of C2H4
+
, C2H5
+
, C2H6
+
and weaker dimers
of (C2H4)(C2H5)
+
, (C2H6)(C2H5)
+
and (C2H6)(C2H6)
+
are observed and their IR frequencies are
reported. For larger clusters the spectra resemble that from neutral clusters. This work is published
in Ref.1.
Lastly Chapter 6 documents the IR spectra in the C-H stretching range of different
carbocations (CXHY
+
; X = 2-4; Y = 1-7), many obtained for the first time. Preliminary analysis of
the spectra is done to lay the groundwork for future analysis. All the IR spectra of carbocations
obtained are unique in their own respect and present large material to the field of ion spectroscopy.
IR spectra of C2H
+
, C3H6
+
, C4H3
+
, C4H4
+
, C4H5
+
, C4H7
+
is reported for the first time in this work.
4
In addition, I have participated in the X-Ray diffraction imaging experiments of helium
droplets at Stanford Linear Accelerator Center and European XFEL. This work is published in
Reference 5,6. I have also worked for the preparation of manuscript for Ref. 7 on evaporation
dynamics of Ag-doped He Droplets.
1.2 List of Publications
1. Erukala, S.; Feinberg, A.; Singh, A.; Moon,C.J.; Choi,M.J.; Vilesov, A. F., Infrared
spectroscopy of carbocations upon electron ionization of ethylene in helium nanodroplets. J.
Chem. Phys. 2022, 156, 204306.
2. Erukala, S.; Feinberg, A.; Singh, A.; Vilesov, A. F., Infrared spectroscopy of carbocations
upon electron ionization of ethylene in helium nanodroplets. J. Chem. Phys. 2021, 155, 084306.
3. Erukala, S.; Verma, D.; Vilesov, A., Rotation of CH3
+
Cations in Helium Droplets. J. Phys.
Chem. Lett 2021, 12, 5105-5109.
4. Verma, D.; Erukala, S.; Vilesov, A., Infrared Spectroscopy of Water and Zundel cations in
Helium Nanodroplets. J. Phys. Chem. A 2020, 124, 6207-6213.
5. Feinberg, A. J.; Verma, D.; O’Connell-Lopez, S. M. O.; Erukala, S.; Tanyag, R. M. P.;
Pang, W.; Saladrigas, C. A.; Toulson, B. W.; Borgwardt, M.; Shivaram, N.; Lin, M.-F.; Haddad,
A. A.; Jäger, W.; Bostedt, C.; Walter, P.; Gessner, O.; Vilesov, A. F., Aggregation of solutes in
bosonic versus fermionic quantum fluids. Science Advances 2021, 7, eabk2247.
6. Verma, D.; O’Connell, S. M. O.; Feinberg, A. J.; Erukala, S.; Tanyag, R. M. P.; Bernando,
C.; Pang, W.; Saladrigas, C. A.; Toulson, B. W.; Borgwardt, M.; Shivaram, N.; Lin, M.-F.; Al
Haddad, A.; Jäger, W.; Bostedt, C.; Walter, P.; Gessner, O.; Vilesov, A. F., Shapes of rotating
normal fluid
3
He versus superfluid
4
He droplets in molecular beams. Phys. Rev. B 2020, 102,
014504.
7. Jones, C. F.; Bernando, C.; Erukala, S.; Vilesov, A. F., Evaporation Dynamics from Ag-
Doped He Droplets upon Laser Excitation. J. Phys. Chem. A 2019, 123, 5859-5865.
5
Chapter 2. Infrared Spectroscopy of Water and Zundel cations in
Helium Nanodroplets
This chapter is based on publication by D.VERMA, S.ERUKALA, AND A.F.VILESOV,
“Infrared Spectroscopy of Water and Zundel cations in Helium Nanodroplets”, J.Phys.Chem.A.,
124, 6207-6213 (2020)
Here, we show that electron impact ionization of helium (He) droplets doped with water
molecules and clusters, yield water and Zundel cations embedded in the droplets consisting of a
few thousand helium atoms. Infrared spectra in the OH-stretching range were obtained using
release of the cations from the droplets upon laser excitation. The spectra in He droplets appear to
have about a factor of 10 narrower bands and similar matrix shifts as compared to those obtained
via tagging with He and Ar atoms. The results confirm the calculated structure of the free Zundel
ion, where the proton is equidistant from the two water units. The effect of the He environment on
the spectra of ions is discussed. The signal shows nonlinear laser pulse energy dependence
consistent with evaporation of the entire droplet upon multiple absorption of infrared photons. This
conclusion is supported by the model calculations of the efficiency of the cations' release versus
laser flux.
2.1 Introduction
2.1.1 Vibrational spectroscopy of molecular ions
Spectroscopy of ions could rarely be done by direct observation of the light absorption
because it is challenging to achieve sufficiently high density of ions. Henceforth, many sensitive
spectroscopic methods have been developed to obtain ion spectra. Herein, we review some of
6
techniques used to obtain vibrational spectra of ions. In the past decade spectroscopy in helium
droplets has joined this field and is making significant contributions.
2.1.1a High Resolution spectroscopy of ions in a discharge
Figure 2.1. Water cooled multiple inlet-outlet discharge cell used to generate ions for gas phase ion
spectroscopy
1
The early measurements of the infrared spectra of simple ions such as HeH
+
, HD
+
employed laser excitation of velocity tuned ion beams followed by indirect mass spectroscopic
detection.
2
However, the resulting spectra were contaminated by the strong bands from the more
abundant neutral precursor molecules. Later, the spectroscopy of molecular ions saw major
developments with the introduction of infrared spectroscopy of ions in glow discharges with
velocity modulation.
1,3
Fig. 2.1 shows an inlet-outlet water cooled discharge tube used by Oka.
1
Ions produced in the discharge plasma are accelerated in axial electric field which produces
doppler shifts and widths in the frequencies. Based on the doppler frequency shift and width of the
line frequency in an oscillating AC plasma, ionic transitions and their corresponding ion masses
were assigned. Spectroscopy in a discharge was used to obtain infrared spectra of the different
small molecular ions such as H3
+
,
4
CH2
+
,
5
CH3
+
,
6
H3O
+
.
7
However, the high resolution spectra are
often complicated by hot bands and signal from neutrals. Therefore, action spectroscopic methods
7
involving tagging with rare gas atoms were developed to overcome these problems as well as to
measure the spectra of larger hydrocarbon ions such as C2H3
+
, C3H3
+
etc.
2.1.1b Matrix isolation spectroscopy.
Figure 2.2. Sampling configuration used in matrix isolation methods, excited neon in discharge ionizes
molecule XY and the ions are deposited onto cryogenic surface.
8
In matrix isolation spectroscopy ESR, optical and infrared absorption measurements are
done on ionic species trapped in dilute solid rare gas solution. Spectra of anions such as C2
-
,
9
CO2
-
,
10
NO2
-11
etc. have been obtained by using alkali metals to stabilize the anions. Number of
halomethyl, halomethane, halobenzene cations produced via photoionization and chemical
ionization were also studied in solid matrices. In addition, cluster ions such as HAr2
+
,
12
O4
-13
were
also studied in matrices. A wider range of molecular ions were studied using discharge sampling
configuration shown in Fig. 2.2. XY molecule diluted with rare gas atoms (Ne/Ar) are ionized
upon collisions with excited rare gas atoms or photoionized by the rare gas resonance radiation
before being frozen into the cryogenic surface at ~ 4 K. Subsequently the IR spectra of the matrix
isolated carbocations such as C3H4
+
,
14
C4H2
+15
etc., were obtained using Fourier transform infrared
8
spectroscopy. However, large matrix shifts were observed in matrix isolation studies of some ions
like H2O
+
,
16
which were assigned to the proton sharing with the rare gas atom. It was also often
difficult to assign the observed spectral features in the absence of mass resolution.
2.1.1c Infrared laser photodissociation
Figure 2.3. a) Schematic of IRPD set up consisting of ion discharge source skimmed and mass selected
with tag atoms prior to laser interaction in octupole ion trap
17
b) the tag dissociates upon vibrational
excitation and the photofragments are mass selected to record the spectra.
Most of the infrared spectra for carbocations were obtained via the infrared
photodissociation (IRPD) of ions tagged with rare gas atoms developed by Y.T Lee.
17
Figure 2.3
shows the general schematic of the experiment on IRPD of ions.
17
Typically the cations and their
complexes are produced in a corona discharge applied close to the nozzle from which a mixture of
precursor gas with rare gases such as argon, neon and helium expands into vacuum. The complexes
of ions with rare gas atoms are mass selected and guided into the octopole ion trap. The trapped
ions undergo laser interaction with a beam of a tunable IR laser. The complexes of ions with rare
gas atom tags dissociate upon infrared excitation; the photofragment ions are mass analyzed and
detected. This technique has been used to obtain the spectra of a large number of ions. However,
the spectra are often complicated by the interaction of the tags with ions as well as with the
9
possibility of different structure of the complexes. The internal temperature of the obtained
complexes was found to be about 100 K, which caused some substantial broadening of the bands.
2.1.1d Vibrational spectroscopy in cryogenic ion traps
Figure 2.4. Schematic of infrared multiphoton dissociation ion spectroscopy using cryogenically cooled
22 pole ion trap to obtain ions at ~5 K.
18
Ions traps are widely used in IRPD spectroscopy to accumulate ions and to increase the
interaction time with the laser radiation. Contemporary experiments often employ cryogenic
multipolar ion traps. Ion temperatures down to 4 K have been achieved using such cryogenic ion
traps in conjugation with collisional buffer He gas to bring the ions to the ground state. Figure 2.4
shows an example of an ion spectroscopic experiments in the group of S. Schlemmer.
18
Ions
produced upon electron impact are mass selected and trapped in cryogenically cooled 22-pole ion
trap where He gas is introduced via pulsed valve to cool the ions as well as to form complexes
with He atoms. Upon laser irradiation and concomitant photodissocation the number of the trapped
complexes decreases. Among the other studies, experiments in a cryogenic trap were used in the
group of M. Johnson to obtained the spectra of H5O2
+
tagged with He atoms at ~ 12K.
19
10
Experiments in cryogenic traps were also used to attain the spectra of free (untagged) ions such as
CH5
+20
by monitoring faster reactivity of vibrationally excited ions.
2.1.2 Vibrational spectroscopy of ions in Helium Droplets
Molecular ions are important intermediates in chemistry of condensed phase and upper
atmosphere as well as in astrochemistry.
21-23
Infrared spectroscopy has been widely used for
interrogation of the structure of ions as well as their reactivity and interaction with solvent species.
Therefore, great strides were done in developing new techniques for spectroscopy of ions as
reviewed in section 2.1.1. The low temperature (~ 0.4 K), liquid state, weak interaction, and high
ionization potential of helium (He) atoms makes helium droplets an ideal matrix for spectroscopy
of ions. He droplet spectroscopy has been extensively applied for neutral species and relies on
depletion of the He droplet beam upon infrared absorption of the embedded species.
24-28
The
depletion technique could not be applied to the embedded ions due to very low fraction of droplets
containing ions. Observation of the ejection of molecular ions from the droplets upon infrared
irradiation by M. Drabbels et. al.
29
has prompted further advances in the field. In the first
experiments, the embedded aniline ions were produced through resonance-enhanced multiphoton
ionization of the embedded molecules.
29
Alternatively, G. von Helden and co-workers produced
ions through electrospray, stored them in an ion trap followed by pickup by the droplet.
30
Employing photoionization and electrospray techniques require more involved apparatus as
compared to He droplet depletion spectroscopic experiment and also imposes limitations on the
kind of ions that could be produced.
Here, we turn our attention to electron impact ionization (EI) which is a reliable and a well-
established technique. It is long known that EI of droplets containing few thousands of He atoms
11
doped with neutral species leads to effective production of molecular ions and various ionic
fragments.
31, 32
Ionization of embedded atoms, A, leads to formation of HenA
+
clusters, with up to
few tens of He atoms. However, the production of the He complexes with molecular ions is much
less efficient, which likely relates to formation of vibrationally excited ions and concomitant
dissociation of He atoms.
31, 32
Few noticeable exceptions include fullerenes, such as C60, as studied
in group of P. Scheier which was found to form a long progression of HenC60
+
containing up to
about 100 attached He atoms.
33
Larger mass of C60 also helps in dramatically reducing interference
from diverse small ions, such as Hen
+
and rest gas species. Laser induced dissociation of the
HenC60
+
was used to study the solvation of C60
+
with atomic resolution.
33
A. Ellis et al. recently
produced complexes of protonated acetic acid tagged with one or two He atoms
34
and protonated
carbon dioxide with up to 14 He atoms
35
upon EI of the doped He droplets containing about 5000
He atoms. The infrared spectra were obtained via depletion infrared laser spectroscopy of the
complexes.
Spectroscopic study of ions through solvation in droplets of few thousand He atoms
appears more favorable than through tagging by few atoms, since the droplets offer a homogeneous
environment with a well-defined temperature of about 0.4 K
36
leading to narrower spectral lines.
Our experimental approach rests on the conjecture that some unknown fraction of ions produced
via EI of the doped droplets remain solvated in the droplets of a few thousand He atoms.
In this work, we show that through EI, ionic species embedded in the droplets can be
formed, making it a convenient technique for producing a wide variety of embedded ions and ionic
clusters. Infrared spectra are obtained by measuring the intensity of the cations released from the
droplets upon laser excitation. Additionally, we report the OH-stretch spectra of water (H2O
+
) and
12
Zundel (H5O2
+
) cations in helium droplets in the 3 μm region. These important cations were chosen
as candidates for validation of this newly developed technique. The spectra are compared with the
earlier results obtained by tagging and followed by discussion on the structure of the ions and their
interaction with the superfluid helium environment. The signal shows nonlinear laser pulse energy
dependence consistent with evaporation of the entire droplet upon multiple absorption of infrared
photons. The results of the model calculations suggest a possible improvement in the performance
of our technique upon moderation of the droplet sizes.
2.2 Experimental setup
Figure 2.5. Schematic of helium droplet setup for molecular ion spectroscopy. He droplets are produced
from a cold pulsed nozzle in the source chamber. The He droplets capture dopants in the pickup chamber
and traverse into the detection chamber equipped with two electron impact ionizers: External and Probe.
The droplets are ionized in the external ionizer and interact with the counter propagating infrared laser beam
once in the ion region of the probe ionizer. The released ions are mass selected by the quadrupole mass
spectrometer.
13
This work builds on modifications to a molecular beam UHV vacuum apparatus, which
has previously been used for spectroscopy of neutral species.
25-28
Figure 2.5 shows the schematics
of the experimental apparatus. He droplets are produced upon expansion of helium gas at
stagnation pressure of P0 = 20 bars and temperature of T0 = 23 K in vacuum through a 1 mm
diameter pulsed nozzle (General Valve series 99) attached to a Sumitomo RDK 408 refrigerator.
37
The nozzle produces pulses of He droplets of about 100 - 200 μs width which is controlled by a
pulse driver (IOTA ONE).
37, 38
Upon collimation by a 2 mm diameter skimmer, He droplets enter
the pickup chamber where they capture water molecules. Going further downstream, the doped
droplets enter the detection chamber that hosts a quadrupole mass spectrometer (QMS) (Extrel
MAX 500) equipped with a standard axial electron impact ionizer, which will be referred to as
probe ionizer. An additional axial ionizer (here referred as external ionizer), same as probe ionizer,
is placed ~ 20 cm upstream from the QMS probe ionizer and has a stack of Einzel lenses for
separation of light and heavy ions.
When the probe ionizer is ON and external ionizer is OFF, the set-up is equivalent to that
as used for depletion spectroscopy of neutral species.
25
This mode is used for aligning of the He
droplet beam, initial adjustment of the pickup pressure, and determination of the flight time of the
droplets from the nozzle to the QMS. For spectroscopy of ions, the external ionizer is ON and
typically set up to an electron energy of 100 eV and emission current of 10 mA. In this mode, the
probe ionizer is OFF, while the ion optics voltages (ion range, extraction etc.) are ON and set to
the same values as during the standard probe operation. Electron impact of the doped droplets in
an external ionizer leads to a large variety of products, such as Hen
+
, free splitter ions of the
dopants, as well as dopant and splitter ions embedded in the droplets. The heavy droplets
14
containing ions continue traversing towards the QMS, whereas the light ions produced post
ionization are rejected by the Einzel lenses, which act as a high pass filter.
The doped ionic droplets are irradiated by a pulsed infrared laser beam when they pass
through the ion range of the probe ionizer. The laser beam is set to counter propagate with the
droplet beam and is focused into the ion range of the probe by a 25 cm focal length lens. Absorption
of several infrared quanta leads to production of free ions, which are then extracted, mass selected
and detected by the QMS. Infrared spectra are recorded by monitoring the gated (~ 10 μs) QMS
signal of the bare ions set at a desired mass (M =18 and 37 for H2O
+
and H5O2
+
, respectively), as
the laser frequency is scanned. The spectra are obtained using a pulsed optical parametric
oscillator-amplifier (Laser Vision, spectral resolution: ~ 0.1 cm
-1
, pulse duration ~ 7 ns, pulse
energy ~ 5 - 8 mJ, repetition rate 20 Hz). The absolute frequency of the laser is calibrated using
the photo-acoustic spectrum of methane and ammonia molecules.
15
2.3 Results and Discussion
2.3.1 Water Ions
Figure 2.6. (a) Spectrum of H
2
O
+
in helium droplet of ~ 7×10
3
atoms as measured at laser pulse energy of
~ 5 mJ/pulse. Measurement time is about 10 min. The origins of the ν 1 and ν 3 bands of the free ions are
marked by green vertical bars, whose heights are proportional to the infrared intensity.
39
(b) Intensity
dependence of v 3 band versus the laser pulse energy (E): Black squares represent the experimental results
while the red curve shows a fit by I = a∙E
3.9
for E < 2 mJ.
Figure 2.6 (a) shows the spectrum of water ions in He droplets. Water molecules were
picked up upon admitting water vapor at 1×10
-6
mbar into the pickup chamber, the pressure
maximizing the laser induced ion signal. The average size of He droplets, NHe ≈ 7000, is estimated
through the Poisson pickup pressure dependence of the ion signal assuming that the pickup cross
section equals to the average geometric cross section of the droplets. An intense peak at 3233.0
cm
-1
is assigned to the asymmetric O-H stretch, v3. A second weaker peak is also seen at ~ 3259.6
cm
-1
, whose assignment is less clear. In free H2O
+
ions, the origins of the symmetric, v1 , and
asymmetric, v3 , bands were found at 3213 cm
-1
and 3259 cm
-1
, respectively, which are shown by
green bars in Figure 2.6 (a).
39
In earlier studies by Dopfer et al.,
40
v1 = 3198 cm
-1
and v3 = 3254
cm
-1
were found in He-H2O
+
complexes. Larger shift of the Δv3 = ~ 26 cm
-1
in He droplets, with
respect to free ions, is likely related to the cumulative effect of the liquid helium environment,
16
where the first solvation shell of the ion is made of ~ 15 He atoms. We did not observe any
significant peaks in the broader survey scans at frequency below 3200 cm
-1
. However, there is a
possibility that such a peak may have been missed due to low intensity owing to non-linear laser
pulse energy of the signal as described in the following.
We have also considered the possibility where the 3259.6 cm
-1
peak is assigned to the v1
band. Reversal of the v1 and v3 bands has been found in Ar2-H2O
+
complexes with Ar atoms bound
to protons (H-bound).
41-43
According to calculations for He-H2O
+
complexes,
40
He atoms are
strongly bound to the protons (H-bound) (De = 520 cm
-1
) whereas the binding is weaker for the p-
bound (De = 230 cm
-1
) and O-bound (De = 60 cm
-1
). Therefore, it is feasible that the primary
spectroscopic unit may be regarded as C2v He2-H2O
+
complex having two H-bound He atoms. It is
interesting to see that in Ar4-H2O
+
complexes the splitting between the v1 and v3 bands diminishes,
which was assigned to the effect of the p-bonding of the additional Ar atoms.
41, 42
In Ne and Ar
matrices, v1 was found at lower frequency than v3, same as in gas phase.
16, 44
Another possible
assignment of the 3259.6 cm
-1
peak is to a combination band of v3 and vibrations of He atoms.
It is known that the spectra of small molecules in helium show rotational structure.
25, 27, 28
In particular, water molecules in helium droplets have similar rotational constants as in free
molecules.
45
The rotational constants of the free water ion are known to be A= 28.0 cm
-1
, B = 12.4
cm
-1
and C = 8.5 cm
-1
.
39
The fact that no rotational structure was observed for water ions indicates
a large reduction of the rotational constants in helium. Assuming, that the primary molecular unit
of an ion concerning the ro-vibrational spectra is He2-H2O
+
, with two He atoms in the H-bound
positions, the rotational constants could be estimated based on the structure of the He-H2O
+
complexes
40
to be A = 1.31 cm
-1
, B = 0.42 cm
-1
and C = 0.32 cm
-1
. The values of the rotational
constants are expected to be significantly lower in He environment. This effect is well documented
17
for neutrals where the rotational constants decrease by about a factor of three, depending on the
particular molecule.
25-28
As a result, the rotational structure of the v3 band in He could not be
resolved in agreement with the experimental observations. The v3 band in Fig. 2.6 (a) has a width
(FWHM) of ~ 2 cm
-1
. This is comparable to the width of ro-vibrational lines of water molecules
in He in the range of 0.3 – 3 cm
-1
,
45
and the width of bands of larger molecules with unresolved
rotational structure in He.
25, 27, 28
Therefore, a factor of ~ 10 stronger interaction of ions with the
nearest neighbor He atoms as compared with neutrals, does not cause any excessive line
broadening.
Figure 2.6 (b) shows the laser pulse energy (E), dependence of the intensity of the v3 band
in Fig. 2.6 (a). E was measured at the entrance window to the vacuum apparatus and varied by
changing of the energy of the Nd:YAG pump. At the highest laser pulse energy of ~ 2.5 mJ, ~ 10
water ions were detected per each pulse. It is seen that the dependence is nonlinear, whereas at low
pulse energy the intensity scales as ~ E
4
. The nonlinear dependence of the intensity distorts the
true infrared intensity ratio of the bands, making weaker bands even weaker. The mechanism of
the ion release from the droplets upon laser irradiation and the E - dependence will be discussed
in Section 2.3.3.
18
2.3.2 Zundel Ions
Figure 2.7. Spectrum of H 5O 2
+
in He droplets (red), compared with the spectra of He-H 5O 2
+
(blue)
19
and
Ar-H 5O 2
+
(black).
46
The band origins of the free ions are marked by green vertical bars,
47
whose heights are
proportional to the infrared intensity.
48
Red trace in Figure 2.7 shows the spectrum of Zundel ion (H5O2
+
), in helium droplets. The
spectrum was measured upon the pickup of several water molecules in the pickup chamber filled
with water vapor at 2×10
-6
mbar. The trace represents a true zero background measurement,
whereas some noise seen in Figure 2.6 (a) is due to a spurious background signal from water from
the rest gas. According to previous computational studies,
48, 49
the proton in Zundel ion has a
minimum potential energy configuration of C2 symmetry and the proton equidistant from the two
19
water moieties, which are staggered by about 100 degrees. Large amplitude motion of the H2O
moieties results in an effective D2d symmetry of the complex.
48
The geometry of the H2O-H
+
units
is close to pyramidal. The H5O2
+
has two symmetric (vs) and two antisymmetric (va) OH stretching
modes.
47
Only the out of phase symmetric stretch has an appreciable infrared intensity. However,
the in phase and out of phase antisymmetric stretches are predicted to have very similar intensity.
48
Accordingly, the two major bands of H5O2
+
in He droplets at 3600.4 cm
-1
and 3678.6 cm
-1
are
assigned to the vs and va stretch of the -OH2 moieties in Zundel ion, respectively with a FWHM of
1.4 cm
-1
and 2.7 cm
-1
, respectively. The bands correspond to parallel and perpendicular transitions,
respectively. While the vs appears as a single band, a shoulder is observed at the low frequency
side of the va band at ~ 3675 cm
-1
and having intensity of ~ 20 % of the main peak. The small
splitting of about 3 cm
-1
may result from different effects, such as, interaction between the va modes
of the two H2O moieties or from a slightly off-center position of the proton in H5O2
+
. The splitting
between the two asymmetric modes is estimated to be less than 1 cm
-1
, whereas the bands should
be degenerate under the effective D2d symmetry.
48
The large difference in the intensity of the two
components of the va band is in disagreement with the results of calculations which predict very
similar intensities.
48
The tunnel splitting associated with the hindered internal rotation of the water
units in H5O2
+
may be another reason for the band splitting as was previously observed by Yeh et.
al.
50
In this case, the intensity ratio of the sub-bands are determined by the population of the nuclear
spin isomers, such as 1:3 for para:ortho modifications of water molecules. The tunnel splitting in
the ground state of H5O2
+
was calculated to be ~ 1 cm
-1
.
48, 50
Tunnel splitting due to internal
rotation in dimers, such as (HF)2 and (H2O)2 in He droplets have been previously documented.
51,
52
This splitting effect due to tunneling was also observed in the spectra of the He-H2O
+
complexes.
40
20
Figure 2.7 also shows the results of previous measurements of the H5O2
+
spectra for
comparison. The band origins of free H5O2
+
ions are shown by green vertical bars at 3608.8 cm
-1
and 3684.4 cm
-1
,
47
whose heights are proportional to the calculated infrared intensities.
48
The blue
and black traces in Figure 2.7 are the results of measurements with He and Ar tagging, respectively.
19, 46
The two bands observed with He tagging are at vs = 3604.3 cm
-1
and va =3687.8 cm
-1
,
respectively.
19
It is seen that the vs and va band positions of the H5O2
+
in helium droplets are
redshifted by ~ 4 cm
-1
(8 cm
-1
) and ~ 9 cm
-1
(6 cm
-1
), respectively, as compared with those in the
He tagged complexes (or free ions). The additional shift in He droplets with respect to He tagging
is small and is just a few wavenumbers larger than the shifts typically observed for molecules in
He droplets.
25, 27
The spectrum in He droplets also shows a factor of ~ 10 narrower bands as
compared with He tagging, which may relate to lower temperature in He droplets as well as to
homogeneous superfluid environment. Besides the shoulder of the va band, the spectrum in He
droplets shows no additional splitting of the bands, which suggests that the shared proton is
symmetrically arranged with respect to the two water moieties. Such symmetry breaking
phenomenon has previously been observed with Ar tagged (black) Zundel ion spectrum in Figure
2.7 which consists of four bands arising from the asymmetric and symmetric stretches of free (va
NB
,
vs
NB
) and Ar bound (va
Ar
, vs
Ar
) H2O units.
46
We did not find any previous calculations for the He-H5O2
+
complexes. Calculations for
Ar-H5O2
+
complexes yield the most favorable binding in the H-bound positions with Ar binding
energy of De ~ 960 cm
-1
,
53
which is smaller than for the Ar-H2O
+
of ~ 2500 cm
-1
.
41
We assume
that the binding energy of He atoms with H5O2
+
is also smaller than with H2O
+
, thus a smaller
effect of the He environment on the spectra.
21
2.3.3 Formation of the Doped Ions and Laser Induced Release of Free Ions.
The results presented in this work show that the ionization of water doped He droplets leads
to formation of diverse embedded ionic species, such as H2O
+
and H5O2
+
. The water cations may
result from ionization of single water molecules, whereas the formation of Zundel cations must
involve ionization of (H2O)3 or larger clusters, which are formed in the interior of the droplets
upon pickup of several water molecules. It follows that He droplets provide a confining force
strong enough to keep some of the ions formed upon EI from leaving the droplet, but weak enough
for product of ion molecular reactions such as OH or possibly OH(H2O)n to leave as it is evidenced
by the observation of the H5O2
+
spectrum in Figure 2.7. The ionization cross section of He atoms
by 100 eV electrons is known to be 3.5×10
-17
cm
2
.
54
Using an estimated electron current flux of
the order of 3×10
-3
A·cm
-2
and time of flight through the ion range of about 20 μs, the ionization
efficiency per He atom is estimated to be about 10
-5
. Therefore, about one in every 20 droplets is
ionized. Although, a rather large mean free path of electrons in liquid helium of about 10 nm may
facilitate the direct ionization of the dopants by electrons, it is more probable that electrons first
produce He
+
ions or He* metastable atoms followed by the creation of ions via charge transfer or
Penning ionization.
32
The ionization and ensuring ion-molecular reactions likely produces a large
fraction of vibrationally excited ions, which cool down upon energy transfer to He droplets and
concomitant evaporation of He atoms. For comparison, the release of 10000 cm
-1
of energy will
lead to evaporation of about 2000 He atoms. During this energy relaxation, some of the ions leave
the droplet. Currently, the fraction of specific ionic products that remain inside the droplets is
unknown; however, it is sufficiently large to obtain a high-quality infrared spectra.
22
In the initial study by Drabbels et. al.,
29
a close to linear dependence of the number of
produced aniline ions from He droplets of about 2000 He atoms vs laser pulse energy was reported.
It was hypothesized that non-thermal ejection of the ions upon IR excitation takes place.
29, 55
Later,
studies
56
found a strongly nonlinear laser energy dependence of the ejection signal of protonated
leu-enkephalin and its 18-crown-6 complex from He droplets of about 20000 atoms upon
excitation by IR free electron laser with up to ~ 40 mJ per macro-pulse. The dominant yield of
bare ions and small yield of Hen attached ions was argued to support the conjecture of the non-
thermal ejection and is incompatible with the thermal evaporation mechanism.
29, 55, 56
Here, we
show that the laser pulse energy dependence of the signal in Figure 2.6 (b) is in good agreement
with the evaporation of all He atoms upon absorption of multiple infrared quanta.
Thermal evaporation of a droplet containing 7000 He atoms requires absorption of about
20 quanta of 3200 cm
-1
. Multiple absorption during the ~ 7 ns laser pulse implies that the
vibrationally excited ions relax to the ground state within the time faster than ~ 400 ps. This is
about a factor of twenty faster than the relaxation time of the ν3 vibration of neutral water
molecules in He droplets of ~ 7 ns as estimated from the laser saturation dependence of the
depletion signal.
45
However, the values of this magnitude seem reasonable in view of the stronger
interaction of the ions with He environment. Most likely the relaxation of the ν3 vibration into the
ground state involves intermediate states, such as ν1, 2ν2 and ν2. Previous spectroscopic studies
indicated fast vibrational relaxation of large amplitude bending and torsional modes with lifetime
of ~ 1 ps.
25, 27
For comparison, the lower limit for lifetime in He-H2O
+
complexes were estimated
as τ1 > 50 ps and τ3 > 150 ps for the ν1 and ν3, bands, respectively.
40
Here, we carry out model calculations for the efficiency of production of bare ions from
He droplets, where the sizes follow a logarithm - normal distribution.
57
The calculations are
23
performed at the maximum of the Lorentz line having FWHM, δν, with infrared intensity of IRI.
We assumed Gaussian beam shape in the ion region of the QMS with the beam waist radius of w.
For a droplet containing N atoms, we assume that the bare ion is produced upon absorption of k
photons when k×ΔN >N. Here, ΔN=3200 cm
-1
/Hevap ≈ 400, is the number of He atoms evaporated
upon absorption of a single 3200 cm
-1
quantum and Hevap (T = 2.7 K) = 7.8 cm
-1
is the evaporation
enthalpy for a single He atom.
58
The temperature of the droplet was estimated to be ~ 2.7 K
considering that 7000 He atoms are evaporated during the 7 ns laser pulse and using saturated
vapor pressure of He.
58
The probability for more than k photons being absorbed at a given laser
flux is calculated using Poisson distribution.
Figure 2.8 shows the results of the model calculations. The ordinate in Figure 4 gives the
efficiency of the ion production, A, which is expressed as an effective cross section area
perpendicular to the droplet beam wherefrom the ions are completely extracted from the droplets.
This efficiency is normalized on the area of the beam waist, πw
2
. The abscissa shows the reduced
laser flux, F = E∙IRI/(δν∙ πw
2
), in which E is the laser pulse energy.
The results of droplets with average size of 7000 atoms are represented by the red curve in
Figure 2.8. It is seen that the efficiency, A, is very small at small values of F after which it rises
approximately linearly at F > 2.5×10
3
mJ·km·mol
-1
·cm
-1
. Measurements in Figure 2.6 (b) show
similar dependence with the efficiency taking off at E > 1 mJ. For the ν3 band of water ions IRI ≈
500 km·mol
-1
,
40
and δν ≈ 5 cm
-1
. The band appears narrower in Figure 2.6 (a) due to the non-
linear laser pulse energy of the signal. The laser beam waist radius w ≈ 0.08 cm is approximated
from the nominal divergence of the laser beam of 4.5 mrad and 8.5 mrad in the horizontal and
vertical directions, respectively, and the focal length of 25 cm. It follows that at E = 1 mJ, F ≈
5000 mJ·km·mol
-1
·cm
-1
. Thus, the laser pulse energy in Figure 2.6 (b) at which the efficiency start
24
rising seems to be a factor of ~ 2 larger than the calculated. The part of the discrepancy is due to
the fact that the values of pulse energy in Fig. 2.6 (b) were measured at the entrance of the vacuum
apparatus. The laser pulse energy in the ion range and the beam waist parameter were not measured
in this work due to lack of capability. In addition, the evaporation of ~ 100 last He atoms should
likely be treated differently because they have larger values of Hevap due to their close proximity
to the ions. The matrix shift of the transition will be smaller for an ion solvated by a small number
of He atoms as compared to that in He droplets and the linewidth may be broadened due to higher
temperature of the clusters, both leading to a smaller absorption cross section for infrared radiation.
Note also that calculations in Figure 2.8 implied instant vibrational relaxation of cations in He
droplets. In case of a finite, but currently unknown relaxation time, τ, the ions remain unavailable
for re-excitation. This effect leads to higher laser flux required for the same effect. Considering
these approximations, we conclude that the measured and calculated laser flux curves are in
reasonable agreement. This agreement lends further support to the thermal evaporation mechanism
of the He droplet upon laser excitation.
The nonlinear laser pulse energy dependence observed in this work and previously in Ref.
56
is inconvenient and may complicate the identification of the bands. Although in principle the
intensities could be corrected using measured or calculated dependence of the signal on the laser
pulse energy, this procedure may be unreliable for weak or congested bands. In particular, the
weaker bands will appear even weaker and may escape the detection. The results in Figure 2.8
shows that the nonlinear effects are much smaller in small droplets containing 2000 atoms, whereas
the initial dependence appears nearly linear in droplets containing average number of 800 He
atoms. However, working with such small droplets is inconvenient, because of the droplet beam
broadening upon pickup. The probability of an ion produced by EI to stay embedded likely
25
decreases with the droplet size. In this work, droplets much smaller than ~ 7000 atoms could not
be produced from our pulsed source, because of a rather abrupt jump in droplet sizes upon decrease
of the nozzle temperature.
37, 38
Possible improvement of the technique may involve production of
ions in larger droplets followed by subsequent moderation of the droplet size, which could be
achieved upon multiple collisions with He atoms at room temperature.
59
Pertaining to ionic
droplets, a better solution would be using an RF ion guide filled with He gas, as recently
demonstrated by Scheier et. al.
60
Figure 2.8. Efficiency of the ion production, A, vs reduced laser flux, F. See text for more details. Red,
blue and green curves are for average droplet sizes of 7000, 2000 and 800 atoms, respectively.
26
2.4 Conclusions
Producing embedded ions by electron impact ionization of the doped droplets greatly
increases the versatility of He droplet experiments. In fact, most of the contemporary He droplet
spectrometers for neutral species involve mass spectrometers and can be upgraded to include ions.
Large variety of the ions and ionic clusters could be formed starting with volatile molecular units,
such as hydrocarbons, ammonia, or water molecules as done in this work. The OH-stretching
spectra of H2O
+
and H5O2
+
in helium droplets are of higher resolution as compared to the
conventional tagging and matrix isolation methods. The ν3 band of H2O
+
has a matrix shift of about
26 cm
-1
towards smaller energy. The spectra of larger ionic species, such as H5O2
+
are close to
those of free species, consistent with weaker binding to the He environment. The obtained spectra
are in agreement with the calculated structure of the free Zundel ion, where the proton is
equidistant from the two water units. The non-linear power dependence of the spectral intensity
on the laser pulse energy indicates that multiple photons are required to set the embedded ions
free, which is broadly in agreement with the thermal evaporation of the entire droplet. The model
calculations of the ion ejection efficiency as a function of laser flux show that for large droplet
sizes, as in this work, a strong non-linear dependence is observed as compared to smaller droplet
sizes, where the dependence is close to linear. Future experiments with small droplet sizes
containing ~ 10
3
atoms may help in eliminating this non-linear effect.
Acknowledgements
This work was supported by the National Science Foundation under Grant CHE-1664990.
The authors thank M. Johnson for making the data available to us for the spectra of Zundel ions
tagged with Ar and He, which are presented in Figure 2.7.
27
2.5 References
1. Oka, T.; Saykally, R. J.; Foth, H.-J.; Hirota, E., Infrared spectroscopy of carbo-ions.
Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical
Sciences 1988, 324, 81-95.
2. Wing, W. H.; Ruff, G. A.; Lamb, W. E.; Spezeski, J. J., Observation of the Infrared
Spectrum of the Hydrogen Molecular Ion HD
+
. Phys. Rev. Lett. 1976, 36, 1488-1491.
3. Gudeman, C. S.; Begemann, M. H.; Pfaff, J.; Saykally, R. J., Velocity-Modulated Infrared
Laser Spectroscopy of Molecular Ions: The v1 Band of HCO
+
. Phys. Rev. Lett. 1983, 50, 727-731.
4. Oka, T., Observation of the Infrared Spectrum of H3
+
. Phys. Rev. Lett. 1980, 45, 531-534.
5. Rosslein, M.; Gabrys, C. M.; Jagod, M. F.; Oka, T., Detection of the Infrared-Spectrum of
CH2
+
. J. Mol. Spectrosc. 1992, 153, 738-740.
6. Crofton, M. W.; Jagod, M. F.; Rehfuss, B. D.; Kreiner, W. A.; Oka, T., Infrared-
Spectroscopy of Carbo-Ions .III. ν3 Band of Methyl Cation CH3
+
. J. Chem. Phys. 1988, 88, 666-
678.
7. Tang, J.; Oka, T., Infrared spectroscopy of H3O
+
: The ν1 fundamental band. J. Mol.
Spectrosc. 1999, 196, 120-130.
8. Jacox, M. E.; Thompson, W. E., The production and spectroscopy of molecular ions
isolated in solid neon. Res. Chem. Intermed 1989, 12, 33-56.
9. Milligan, D. E.; Jacox, M. E., Studies of the Photoproduction of Electrons in Inert Solid
Matrices. The Electronic Spectrum of the Species C2
−
. J. Chem. Phys. 1969, 51, 1952-1955.
10. Jacox, M. E.; Milligan, D. E., Vibrational spectrum of CO2
−
in an argon matrix. Chem.
Phys. Lett. 1974, 28, 163-168.
11. Milligan, D. E.; Jacox, M. E.; Guillory, W. A., Infrared Spectrum of the NO2
−
Ion Isolated
in an Argon Matrix. J. Chem. Phys. 1970, 52, 3864-3868.
28
12. Milligan, D. E.; Jacox, M. E., Infrared spectroscopic evidence for the stabilization of HArn
+
in solid argon at 14 K. J. Mol. Spectrosc. 1973, 46, 460-469.
13. Jacox, M. E.; Milligan, D. E., Spectrum and structure of the O3
-
and O4
-
anions isolated in
an argon matrix. Chem. Phys. Lett. 1972, 14, 518-521.
14. Liu, M.-C.; Chen, S.-C.; Chin, C.-H.; Huang, T.-P.; Chen, H.-F.; Wu, Y.-J.,
Photoisomerization and Infrared Spectra of Allene and Propyne Cations in Solid Argon. J. Phys.
Chem. Lett 2015, 6, 3185-3189.
15. Szczepanski, J.; Wang, H.; Jones, B.; Arrington, C. A.; Vala, M. T., Infrared absorption
spectroscopy of diacetylene ions trapped in solid argon. Phys. Chem. Chem. Phys. 2005, 7, 738-
742.
16. Forney, D.; Jacox, M. E.; Thompson, W. E., The Vibrational Spectra of Molecular Ions
Isolated in Solid Neon. X. H2O
+
, HDO
+
, and D2O
+
. J. Chem. Phys. 1993, 98, 841-849.
17. Yeh, L. I.; Price, J. M.; Lee, Y. T., Infrared spectroscopy of the pentacoordinated
carbonium ion C2H7
+
. J. Am. Chem. Soc. 1989, 111, 5597-5604.
18. Asvany, O.; Brunken, S.; Kluge, L.; Schlemmer, S., COLTRAP: A 22-pole ion trapping
machine for spectroscopy at 4 K. Applied Physics B-Lasers and Optics 2014, 114, 203-211.
19. Johnson, C. J.; Wolk, A. B.; Fournier, J. A.; Sullivan, E. N.; Weddle, G. H.; Johnson, M.
A., Communication: He-tagged Vibrational Spectra of the SarGlyH
+
and H
+
(H2O)2,3 Ions:
Quantifying Tag effects in Cryogenic Ion Vibrational Predissociation (CIVP) Spectroscopy. J.
Chem. Phys. 2014, 140, 221101.
20. Asvany, O.; Yamada, K. M. T.; Brunken, S.; Potapov, A.; Schlemmer, S., Experimental
ground-state combination differences of CH5
+
. Science 2015, 347, 1346-1349.
21. Agmon, N.; Bakker, H. J.; Campen, R. K.; Henchman, R. H.; Pohl, P.; Roke, S.; Thämer,
M.; Hassanali, A., Protons and Hydroxide Ions in Aqueous Systems. Chem. Rev. 2016, 116, 7642-
7672.
22. Shuman, N. S.; Hunton, D. E.; Viggiano, A. A., Ambient and Modified Atmospheric Ion
Chemistry: From Top to Bottom. Chem. Rev. 2015, 115, 4542-4570.
29
23. Tielens, A. G. G. M., The Physics and Chemistry of the Interstellar Medium. Cambridge
University Press: Cambridge, 2005.
24. Tanyag, R. M. P.; Jones, C. F.; Bernando, C.; O'Connell, S. M. O.; Verma, D.; Vilesov, A.
F., Experiments with large superfluid helium droplets. In Cold chemistry: Molecular scattering
and reactivity near absolute zero, Dulieu, O.; Osterwalder, A., Eds. Royal Society of Chemistry:
Cambridge, 2017; pp 401-455.
25. Toennies, J. P.; Vilesov, A. F., Superfluid Helium Droplets: A Uniquely Cold Nanomatrix
for Molecules and Molecular Complexes. Angewandte Chemie-International Edition 2004, 43,
2622-2648.
26. Callegari, C.; Ernst, W. E., Helium Droplets as Nanocryostats for Molecular Spectroscopy
- from the Vacuum Ultravoilet to the Microwave regime. In Handbook of High-resolution
Spectroscopy, Quack, M.; Merkt, F., Eds. John Wiley & Sons, Ltd.: 2011; pp 1551-1594.
27. Choi, M. Y.; Douberly, G. E.; Falconer, T. M.; Lewis, W. K.; Lindsay, C. M.; Merritt, J.
M.; Stiles, P. L.; Miller, R. E., Infrared Spectroscopy of Helium Nanodroplets: Novel Methods for
Physics and Chemistry. Int Rev Phys Chem 2006, 25, 15-75.
28. Verma, D.; Tanyag, R. M. P.; O'Connell, S. M. O.; Vilesov, A. F., Infrared Spectroscopy
in Superfluid Helium Droplets. Advances in Physics-X 2018, 4, 1553569.
29. Smolarek, S.; Brauer, N. B.; Buma, W. J.; Drabbels, M., IR Spectroscopy of Molecular
Ions by Nonthermal Ion Ejection from Helium Nanodroplets. J. Am. Chem. Soc. 2010, 132, 14086-
14091.
30. Bierau, F.; Kupser, P.; Meijer, G.; von Helden, G., Catching Proteins in Liquid Helium
Droplets. Phys. Rev. Lett. 2010, 105, 133402.
31. Lewerenz, M.; Schilling, B.; Toennies, J. P., Successive capture and coagulation of atoms
and molecules to small clusters in large liquid-helium clusters. J. Chem. Phys. 1995, 102, 8191-
8207.
32. Mauracher, A.; Echt, O.; Ellis, A. M.; Yang, S.; Bohme, D. K.; Postler, J.; Kaiser, A.;
Denifl, S.; Scheier, P., Cold physics and chemistry: Collisions, ionization and reactions inside
helium nanodroplets close to zero K. Physics Reports-Review Section of Physics Letters 2018, 751,
1-90.
30
33. Kuhn, M.; Renzler, M.; Postler, J.; Ralser, S.; Spieler, S.; Simpson, M.; Linnartz, H.;
Tielens, A. G. G. M.; Cami, J.; Mauracher, A.; Wang, Y.; Alcami, M.; Martin, F.; Beyer, M. K.;
Wester, R.; Lindinger, A.; Scheier, P., Atomically Resolved Phase Transition of Fullerene Cations
Solvated in Helium Droplets. Nat.Comm. 2016, 7, 13550.
34. Davies, J. A.; Besley, N. A.; Yang, S. F.; Ellis, A. M., Probing Elusive Cations: Infrared
Spectroscopy of Protonated Acetic Acid. J. Phys. Chem. Lett. 2019, 10, 2108-2112.
35. Davies, J. A.; Besley, N. A.; Yang, S.; Ellis, A. M., Infrared spectroscopy of a small ion
solvated by helium: OH stretching region of HeN−HOCO
+
. J. Chem. Phys. 2019, 151, 194307.
36. Hartmann, M.; Miller, R. E.; Toennies, J. P.; Vilesov, A., Rotationally resolved
spectroscopy of SF6 in liquid-helium clusters - A molecular probe of cluster temperature. Phys.
Rev. Lett. 1995, 75, 1566-1569.
37. Verma, D.; Vilesov, A. F., Pulsed Helium Droplet Beams. Chem. Phys. Lett. 2018, 694,
129-134.
38. Slipchenko, M. N.; Kuma, S.; Momose, T.; Vilesov, A. F., Intense Pulsed Helium Droplet
Beams. Rev. Sci. Intrum. 2002, 73, 3600-3605.
39. Huet, T. R.; Pursell, C. J.; Ho, W. C.; Dinelli, B. M.; Oka, T., Infrared Spectroscopy and
Equilibrium Structure of H2O
+
(X ̃
2
B1). J. Chem. Phys. 1992, 97, 5977-5987.
40. Roth, D.; Dopfer, O.; Maier, J. P., Intermolecular Potential Energy Surface of the Proton-
Bound H2O
+
– He dimer: Ab Initio Calculations and IR Spectra of the O–H Stretch Vibrations.
Phys. Chem. Chem. Phys. 2001, 3, 2400-2410.
41. Dopfer, O., Microsolvation of the Water Cation in Argon: I. Ab Initio and Density
Functional Calculations of H2O
+
−Arn (n = 0−4). J. Phys. Chem. A 2000, 104, 11693-11701.
42. Dopfer, O.; Roth, D.; Maier, J. P., Microsolvation of the Water Cation in Argon: II.
Infrared Photodissociation Spectra of H2O
+
−Arn (n = 1−14). J. Phys. Chem. A 2000, 104, 11702-
11713.
43. Wagner, J. P.; McDonaldII, D. C.; Duncan, M. A., Near-infrared Spectroscopy and
Anharmonic Theory of the H2O
+
Ar1,2 Cation Complexes. J. Chem. Phys. 2017, 147, 104302.
31
44. Zhou, H.; Yang, R.; Jin, X.; Zhou, M., Infrared Spectra of the OH
+
and H2O
+
Cations
Solvated in Solid Argon. J. Phys. Chem. A 2005, 109, 6003-6007.
45. Kuyanov, K. E.; Slipchenko, M. N.; Vilesov, A. F., Spectra of the ν1 and ν3 Bands of Water
Molecules in Helium Droplets. Chem. Phys. Lett. 2006, 427, 5-9.
46. Hammer, N. I.; Diken, E. G.; Roscioli, J. R.; Johnson, M. A.; Myshakin, E. M.; Jordan, K.
D.; McCoy, A. B.; Huang, X.; Bowman, J. M.; Carter, S., The Vibrational Predissociation Spectra
of the H5O2
+.
RGn (RG=Ar,Ne) Clusters: Correlation of the Solvent Perturbations in the Free OH
and Shared Proton Transitions of the Zundel Ion. J. Chem. Phys. 2005, 122, 244301.
47. Yeh, L. I.; Okumura, M.; Myers, J. D.; Price, J. M.; Lee, Y. T., Vibrational Spectroscopy
of the Hydrated Hydronium Cluster Ions H3O
+
.(H2O)n (n = 1, 2, 3). J. Chem. Phys. 1989, 91, 7319-
7330.
48. Vendrell, O.; Gatti, F.; Meyer, H.-D., Full Dimensional (15-dimensional) Quantum-
Dynamical Simulation of the Protonated Water dimer. II. Infrared Spectrum and Vibrational
Dynamics. J. Chem. Phys. 2007, 127, 184303.
49. Huang, X.; Braams, B. J.; Bowman, J. M., Ab Initio Potential Energy and Dipole Moment
Surfaces for H5O2
+
. J. Chem. Phys. 2005, 122, 044308.
50. Yeh, L. I.; Lee, Y. T.; Hougen, J. T., Vibration-Rotation Spectroscopy of the Hydrated
Hydronium Ions H5O2
+
and H9O4
+
. J. Mol. Spectrosc. 1994, 164, 473-488.
51. Kuyanov-Prozument, K.; Choi, M. Y.; Vilesov, A. F., Spectrum and Infrared Intensities of
OH-stretching Bands of Water Dimers. J. Chem. Phys. 2010, 132, 014304.
52. Douberly, G. E.; Miller, R. E., The Growth of HF Polymers in Helium Nanodroplets:
Probing the Barriers to Ring Insertion. J. Phys. Chem. B 2003, 107, 4500-4507.
53. Douberly, G. E.; Walters, R. S.; Cui, J.; Jordan, K. D.; Duncan, M. A., Infrared
Spectroscopy of Small Protonated Water Clusters, H
+
(H2O)n (n = 2−5): Isomers, Argon Tagging,
and Deuteration. J. Phys. Chem. A 2010, 114, 4570-4579.
54. Kim, Y.-K.; Rudd, M. E., Binary-encounter-dipole model for electron-impact ionization.
Phys. Rev. A 1994, 50, 3954-3967.
32
55. Zhang, X. H.; Brauer, N. B.; Berden, G.; Rijs, A. M.; Drabbels, M., Mid-infrared
spectroscopy of molecular ions in helium nanodroplets. J. Chem. Phys. 2012, 136, 044305.
56. González Flórez, A. I.; Ahn, D. S.; Gewinner, S.; Schollkopf, W.; von Helden, G., IR
Spectroscopy of Protonated Leu-enkephalin and its 18-crown-6 Complex Embedded in Helium
Droplets. Phys. Chem. Chem. Phys. 2015, 17, 21902-21911.
57. Lewerenz, M.; Schilling, B.; Toennies, J. P., A new scattering deflection method for
determining and selecting the sizes of large liquid clusters of
4
He. Chem. Phys. Lett. 1993, 206,
381-387.
58. Donnelly, R. J.; Barenghi, C. F., The observed properties of liquid helium at the saturated
vapor pressure. J. Phys. Chem. Ref. Data 1998, 27, 1217-1274.
59. Gomez, L. F.; Loginov, E.; Sliter, R.; Vilesov, A. F., Sizes of large He droplets. J. Chem.
Phys. 2011, 135, 154201.
60. Tiefenthaler, L.; Ameixa, J.; Martini, P.; Albertini, S.; Ballauf, L.; Zankl, M.; Goulart, M.;
Laimer, F.; Haeften, K. v.; Zappa, F.; Scheier, P., An intense source for cold cluster ions of a
specific composition. Rev. Sci. Intrum. 2020, 91, 033315.
33
Chapter 3. Rotation of CH3
+
cations in Helium Droplets
This chapter is based on publication by S.ERUKALA, D.VERMA AND A.F.VILESOV,
“Rotation of CH3
+
cations in Helium Droplets”, J.Phys.Chem.Lett., 12, 5105-5109 (2021)
Observation of free rotation of molecules in helium droplets enabled microscopic study of
interaction of quantum rotors with superfluid environment at T= 0.4 K. This work extends studies
of rotation in helium to molecular cations, such as methenium, CH3
+
. The spectrum of the v3 band
of CH3
+
around 3130 cm
-1
has three prominent peaks assigned to the rotational structure of the
band. While the free CH3
+
is an oblate top, in helium it behaves as a prolate top. This effect is
ascribed to strong binding of two He atoms along the figure axis of the ion. Our results indicate
that the other He atoms within the first solvation shell remain fluxional in disparity with the widely
accepted model of a rigid He "snowball" surrounding ions.
3.1 Introduction
It has long been proposed that a positive ion in liquid helium is surrounded by a so-called
Atkins snowball
1
which is often seen as a shell of solid He with radius of ~ 0.7 nm consisting of
an atomic ion A
+
compressed by induction forces and containing up to about 40 He atoms. The
notion of the snowball is consistent with the increase of the effective mass of A
+
in liquid helium
2
and formation of A
+
HeN clusters upon ionization of doped He droplets.
3
However, no microscopic
experimental studies of the snowball structure and dynamics were forthcoming. While previous
works mostly involved atomic cations, study of the rotation of molecular ions may shed some light
on the interaction with its He solvation shell, similar to previous studies on rotation of neutral
molecules in helium droplets.
4-7
He atom binding to molecules such as CH4
8
or OCS
9
is rather
34
weak of the order 50 cm
-1
. It was found that even He atoms in the first solvation shell of molecules
participate in the superfluid exchange with more distant atoms.
10-12
Therefore a picture of a
molecule immersed in a homogeneous superfluid presents a good approximation.
13
It is desirable
to extend the studies to molecular species that interact stronger with He atoms which may shed
some light on the gradual quenching of the exchange and localization of He atoms. In this work
we explore if the rotation of molecular cations in He is feasible and study the effect of the cation-
He interaction on the rotational spectrum. Similar to the neutrals, the cations are expected to reside
in the interior of the droplets, however, He interaction with cations is a factor of about ten stronger
and calculations yield localization of He atoms in the first solvation shell of the ions, such as in
Li
+
-He12 cluster.
14
Accordingly, the rotational motion of ions may be impaired to a larger extent
as compared with neutrals. Therefore, light symmetric ions appear good candidates for such
studies.
In this work, we report on the measurements of the spectrum of the antisymmetric stretch
v3 band of CH3
+
ions in helium droplets. The spectrum reveals partially resolved rotational
structure assigned to transitions from K" = 0 and 1 states. It appears that the moment of inertia of
CH3
+
around the C3 axis in He is about 15% larger than in free molecules. On the other hand, the
moment of inertia around the perpendicular axis increases by a factor of ~ 50, turning the rotor
from an oblate to a prolate top. We conjectured that due to strong attraction along the C3 axis the
spectrum corresponds to a new molecular spectroscopic unit: He-CH3
+
-He that rotates freely in
superfluid He droplet.
35
3.2 Experimental parameters
The experimental apparatus used for measuring rotational spectra of CH3
+
is similar to that
described in Chapter 2. Helium droplet pulses of ~ 200 μs are produced upon expansion of helium
gas at stagnation pressure of P0 = 20 bar and temperature of T0 = 23 K in vacuum through a 1 mm
diameter pulsed nozzle (General Valve series 99) attached to a Sumitomo RDK 408 refrigerator.
Upon collimation by a 2 mm diameter skimmer, He droplets enter the pickup chamber where they
capture methane molecules. Further downstream, the doped droplets enter the detection chamber
that hosts a quadrupole mass spectrometer (QMS) (Extrel MAX 500) with an additional axial
external ionizer placed ~ 20 cm upstream from the ion range of the QMS. The external ionizer is
typically set up to an electron energy of 100 eV and emission current of 10 mA. Upon ionization
the heavy droplets containing ions continue traversing towards the QMS, whereas the light ions
are rejected by the Einzel lenses which acts as a high pass filter. Infrared spectra are recorded by
monitoring the gated (~ 10 μs) QMS signal of the bare ions set at a desired mass of 15 au for CH3
+
.
The spectra are obtained using a pulsed optical parametric oscillator-amplifier (Laser Vision,
spectral resolution: ~ 0.1 cm
-1
, pulse duration ~ 7 ns, pulse energy ~ 5 - 8 mJ, repetition rate
20 Hz).
36
3.3 Results and Discussion
Figure 3.1. (a) Stick spectrum of the v 3 band of free CH 3
+
with transitions from K" = 0, 1 based on
spectroscopic constants in Ref.
15
. The spectral lines are denoted by (J', K') (J", K"). The heights of the
bars give the calculated relative intensity of the lines assuming 1:1 population ratio of the K"= 0 and 1
states. (b) Spectrum of the v 3 band of CH 3
+
in helium droplets. The transitions with unresolved J- structure
are denoted by K' K". (c) Simulated spectrum of CH 3
+
in helium with modified rotational constants as
described in the text.
37
Figure 3.2. CH 3
+
signal intensity vs pickup pressure of CH 4 recorded with laser frequency tuned to the
maximum of the v 3 band of CH 3
+
in He droplets at 3131.6 cm
-1
. The red curve represents the fit of the data
points by Poisson probability for capture of a single molecule per droplet.
Figure 3.1 (b) shows the spectrum of the v3 (E'← A1') perpendicular band of CH3
+
in helium
droplets. The band appears as a triplet with maxima at 3124.7 cm
-1
, 3131.6 cm
-1
and 3137.6 cm
-1
.
The assignment of the band is based on the close proximity of the frequencies to the v3 band origin
of CH3
+
in the gas phase at 3107.9 cm
-1
.
15
No other bands were found in the range of
2500 - 3300 cm
-1
in agreement with the expectations. CH3
+
has a totally symmetric v1 vibration
with calculated frequency at ~ 2917 cm
-1
which has no infrared intensity.
16
The out of plane v2
vibration band at 1360 cm
-1
17
is out of the range of the present experiment. The spectrum was
recorded at methane pickup pressure of ~ 1 × 10
-6
mbar. The dependence of CH3
+
signal on
methane pickup pressure is shown in Figure 3.2. The red curve in Figure 3.2 represents a fit to the
38
data by a Poisson dependence for capture of single molecules per droplet. Similar Poisson
dependences were previously obtained for neutral dopants.
4-6
The good agreement of the
experimental points and the fit indicates that the CH3
+
ions are produced upon electron impact
ionization of the droplets containing single CH4 molecules, whereas the hydrogen atoms produced
upon the ionization leave the droplets. The average size of He droplets, NHe ≈ 7000, is estimated
from the Poisson pickup pressure dependence of the ion signal in Fig. 3.2 with assumption that the
CH4 pickup cross section equals to the average geometric cross section of the droplets.
We assign the three peaks in the spectrum of CH3
+
in Figure 3.1(b) to the ro-vibrational
structure of the v3 band. Free CH3
+
ion has a planar structure with D3h symmetry.
15
Detailed
discussion of the symmetry of the rotational-vibrational levels can be found in Ref.
18
In the ground
vibrational state, A1' with K" = 0, due to Fermi statistics of protons only the levels with odd J-
values occur with the lowest (J", K") level being (1, 0). CH4 is known to have three spin isomers
I = 0,1,2 with population ratio in He droplets of 2:9:5, respectively.
19
Removal of a proton and two
electrons produces CH3
+
with I = 1/2 and 3/2 with population ratio of 1:1.
20
Therefore, the K" =
0 (A) and K" = 1 (E) states of CH3
+
are expected to have population ratio of 1:1. Same population
ratio was found previously for NH3 (C3v) molecules in He droplets,
21
where it corresponds to the
population ratio of the A- and E-manifolds of levels at high temperature. Therefore, the spectrum
of a free CH3
+
at low temperature should contain transitions from the lowest (1, 0) and (1, 1)
rotational levels of the ground vibrational state, as shown in Figure 3.1(a). The presented
calculations employed rotational constants for free CH3
+
B = 9.272 cm
-1
and C(1-ζ) = 4.05 cm
-1
from Ref.
15
and intensity Hö nl-London factors for a perpendicular band.
18
Here ζ is the Coriolis
coupling constant within the E' excited state which could not be determined. It is seen that the
spectrum derived from using the gas phase rotational constants (Figure 3.1 (a)) bears no
39
resemblance to the spectrum obtained in He droplets (Figure 3.1 (b)). This suggests a large change
in the rotational constants of CH3
+
in helium with respect to the gas phase due to strong interaction
of cations with He atoms.
Spectra of complexes of CH3
+
with one and two He atoms were studied in groups of O.
Dopfer and S. Schlemmer using pulsed expansion and cryogenic ion traps, respectively.
16, 22-24
The
spectrum of CH3
+
with two He atoms was assigned to a prolate symmetric top. The experimental
results and quantum chemical calculations indicated He atoms in the complexes residing along the
C
3
axis. Our results are consistent with CH3
+
in He being a prolate symmetric top. Figure 3.1(c)
shows the calculated spectrum of a symmetric top with A = 3.4 cm
-1
and B = 0.1 cm
-1
constants
and v3 = 3128.05 cm
-1
. In order to account for the non-equidistant peaks, the centrifugal distortion
constant of DKK = 0.04 cm
-1
was introduced. The rotational constants in the ground and excited
states were assumed to be the same. The ro-vibrational lines were taken to have a Gaussian shape
with the width of 0.4 cm
-1
(FWHM). The calculated spectrum appears to be in a good agreement
with the experimental one in Figure 3.1 (b). Some discrepancy in the relative intensity of the sub-
bands is probably caused by nonlinear laser pulse energy dependence of the intensity. This is
related to absorption of multiple photons required to evaporate the entire droplet content to get the
free ions, as discussed in more details previously.
25
This effect makes weaker (01) and (21)
sub-bands even weaker with respect to the stronger (1 0) sub-band. The rotational constant B
could not be reliably determined. The widths of the bands in spectrum in Fig. 3.1 (c) decrease by
less than 0.3 cm
-1
if smaller values of B were taken. Thus, the value of 0.1 cm
-1
gives an
approximate upper bound of the constant. Good agreement of the measured and calculated spectra
indicates that CH3
+
in helium has the rotational spectrum of a prolate symmetric top. Accordingly,
40
the three peaks at 3124.734 cm
-1
, 3131.58 cm
-1
and 3137.66 cm
-1
are assigned to unresolved ΔK =
(0-1), (1-0) and (2-1) sub bands.
Table 3.1. Molecular constants of the v 3 vibrational state of CH 3
+
gas phase, in He droplets and in CH 3
+
-
He 2 complexes. All values are in units of cm
-1
.
* For gas phase, A in the table stands for rotational constant C
Table 3.1 compares the molecular constants of CH3
+
in the gas phase, in He droplets and
in CH3
+
– He2 complexes. Note that essentially the determined values for rotation around C3 axis
correspond to A(1-ζ). Here, we assume that the Coriolis splitting constant ζ does not change in He
as was previously discussed for neutral symmetric rotors, such as SF6, NH3, CH4 and C2H6.
19, 21,
26, 27
It is seen that the rotational constant about the C3-axis is smaller by about 15% than in free
CH3
+
. The decrement is somewhat larger than ~ 5% as seen in NH3
21
and CH4,
19
but similar to
16% in C2H6
27
suggesting that He atoms surrounding the H-atoms of these neutral species and
CH3
+
likely remain fluid. On the other hand, the rotational constant (B) around the axis
perpendicular to the C3 axis decreases dramatically from 9.2 cm
-1
to about or less than 0.1 cm
-1
.
The change of this magnitude was not previously observed in neutral rotors and likely indicates
that some He atoms of the CH3
+
environment rotate rigidly with it. The binding energies, De , for
p-bound and H-bound He atoms in the CH3
+
– He complex were calculated to be ~ 700 cm
-1
and
~ 100 cm
-1
, respectively.
23
The long-range attraction of He by CH3
+
is dominated by induction
forces. At small distance upon approach along the C3 axis, the system undergoes electron transfer
Constant CH3
+
gas phase*
15
CH3
+
in He Droplets CH3
+
-He2
24
v3- A 3107.85 3128.1 3121.31
B 9.272 ≤0.1 0.39
A(1-) 4.045 3.4 4.35
DKK 0.0004 0.04 -
41
from the He atom into the vacant and electrophilic 2pz orbital of the C atom. The charge transferred
is small at the equilibrium geometries of CH3
+
–He of about 0.04e but is substantial for heavier rare
gas atoms such as ≈ 0.3e for Ar.
23
On the other hand, when He atom approaches in the plane of
symmetry of CH3
+
(planar H-bound) the binding is mainly due to induction forces, which are much
weaker. Therefore, in distinction to neutral molecule-He complexes, the bonding mechanism and
its strength strongly depend on the relative orientation of the combining species such as CH3
+
cation and He atom.
23
For comparison, the binding is much weaker ~ 20 - 40 cm
-1
in CH4 – He.
8
Therefore, it is conceivable that the CH3
+
with two attached He atoms on the C3 axis constitute an
effective rotor in liquid helium. The rotational constant B for end-over-end rotation of free He –
CH3
+
– He complex was measured to be 0.39 cm
-1
.
24
The complex represents a heavy rotor, whose
rotational constant in the droplet will be further reduced due to interaction with He atoms of the
environment. The reduction of the rotational constants in He by a factor of ~ 2 - 5 was previously
reported for different heavy molecules in He.
4-6
This reduction reflects the response of the He
atoms of the environment to the rotation of a prolate body.
10-13
In the crude approximation this
response corresponds to the backflow of the He atoms of the environment, which can be accounted
for by some additional moment of inertia.
28
Therefore, the estimated rotational constant of B ≈ 0.1
cm
-1
for CH3
+
in the droplets seems reasonable. Accordingly, our results could be explained by a
rotating rigid He- CH3
+
- He complex, whereas the other He atoms in the vicinity remain fluxional.
Large value of the centrifugal distortion constant DKK ~ 0.04 cm
-1
for CH3
+
in He is also in
line with previous observation in neutral rotors. Its value was estimated to be ~ 0.03 cm
-1
in CH4,
~ 0.04 cm
-1
in SiH4 and ~ 0.02 cm
-1
19
in C2H6,
27
orders of magnitude larger than in corresponding
free molecules and assigned to the effect of interaction with liquid helium environment.
42
The results in Table 3.1 also show that the vibrational frequency of the CH 3
+
shifts by
~ 14 cm
-1
upon addition of two most strongly bound He atoms. In He droplets the high frequency
shift amounts to ~ 20 cm
-1
. In comparison, the vibrational frequency of the neutral molecules in
He droplets usually have a few wavenumbers of low frequency shift.
4-6
Small low frequency shift
of the bands in He droplets was also observed for Zundel ions (H5O2
+
) in He droplets.
25
The high
frequency shift in complexes of CH3
+
with rare gas atoms was assigned to the contraction of the
C–H bonds upon partial filling of the 2pz orbital of C-atoms.
23
As in our previous studies of H2O
+
and H5O2
+
in helium droplets, the width of the spectral
features was found to be ~ 2 cm
-1
(FWHM). This is comparable to the width of ro-vibrational lines
of water molecules in He in the range of 0.3 – 3 cm
-1
,
29
and the width of bands of larger molecules
with unresolved rotational structure in He.
4-6
In the previous works, the width was often assigned
to the lifetime broadening due to ro-vibrational relaxation. Unresolved rotational structure of the
K-sub-bands could also contribute to the observed broadening. It is seen that the (21) band
appears a factor of about two broader as compared with the (01) band in Figure 3.1 (b). This is
again in agreement with the previous observations in molecules such as CH4,
19
HF,
30
NH3
21
and
H2O
29
where broader lines were observed for transitions into higher rotational states of the upper
vibrational level, consistent with the effect of rotational relaxation.
3.4 Conclusion
Here we presented the first experimental observation of the rotational structure of cations
in liquid helium. Previous extensive studies of neutral species in He droplets have shown that the
spectra could be described by introducing effective rotational constants with values close to or up
to a factor of about three smaller than for free molecules. In comparison, the effect is larger in
43
CH3
+
and is manifested dramatically different for rotation around the C3 axis and perpendicular to
it, where the rotational constants decrease by about a factor of 1.2 and 50, respectively. This
unusually large disparity is explained by a strong and anisotropic interaction of CH 3
+
with He
atoms. Strong interaction along the C3 axis draws two He atoms to the complex, making an
effective He-CH3
+
-He rotor. On the other hand, the other atoms in the first solvation shell of CH3
+
likely continue participating into the exchange with more distant He atoms of the superfluid
droplet. This is in disagreement with a commonly used notion of a snowball of solid He shells
around the cations. The existence of such a crystalline shell around CH 3
+
would cause some
splitting of the rotational energy levels, which was not observed in this work within the width of
the spectral peaks of CH3
+
. The width of the peaks may partially be due to unresolved rotational
structure of the sub-bands. In the future it would be interesting to study cations of T d symmetry,
such as CH4
+
and NH4
+
, which likely remain as effective spherical tops in helium.
This work also shows that nuclear spin remains conserved upon formation and cooling of
ions in liquid helium, similar to previous observations for neutral species. This opens an
opportunity for extending the study of the rotation in helium to various prototypical symmetric
cations such as CH4
+
, C2H2
+
, NH4
+
, H3O
+
etc. to study systematically the effect of solvation in
superfluid helium. In species, such as H3O
+
it would also be interesting to see how the inversion
splitting is affected by the solvation.
We have also demonstrated that in addition to previously reported H2O
+
and H5O2
+
cations,
25
the embedded carbo-cations could also be produced by electron impact ionization of
neutral precursors. It seems that this technique could straightforwardly be extended for preparation
44
and study of diverse C-, N-, and O- based cations of different sizes, which act as important
intermediates in chemistry in condensed and gas phase.
3.5 References
1. Atkins, K. R., Ions in Liquid Helium. Phys. Rev. 1959, 116, 1339-1343.
2. Glaberson, W. I.; W. Johnson, W., Impurity ions in liquid helium. J. Low Temp. Phys.
1975, 20, 313-338.
3. González-Lezana, T.; Echt, O.; Gatchell, M.; Bartolomei, M.; Campos-Martínez, J.;
Scheier, P., Solvation of ions in helium. Int Rev Phys Chem 2020, 39, 465-516.
4. Toennies, J. P.; Vilesov, A. F., Superfluid Helium Droplets: A Uniquely Cold Nanomatrix
for Molecules and Molecular Complexes. Angewandte Chemie-International Edition 2004, 43,
2622-2648.
5. Choi, M. Y.; Douberly, G. E.; Falconer, T. M.; Lewis, W. K.; Lindsay, C. M.; Merritt, J.
M.; Stiles, P. L.; Miller, R. E., Infrared Spectroscopy of Helium Nanodroplets: Novel Methods for
Physics and Chemistry. Int Rev Phys Chem 2006, 25, 15-75.
6. Verma, D.; Tanyag, R. M. P.; O'Connell, S. M. O.; Vilesov, A. F., Infrared Spectroscopy
in Superfluid Helium Droplets. Advances in Physics-X 2018, 4, 1553569.
7. McKellar, A. R. W.; Xu, Y. J.; Jager, W., Spectroscopic exploration of atomic scale
superfluidity in doped helium nanoclusters. Phys. Rev. Lett. 2006, 97, 183401.
8. Calderoni, G.; Cargnoni, F.; Martinazzo, R.; Raimondi, M., Potential energy surface,
bound states, and rotational inelastic cross sections of the He-CH4 system: A theoretical
investigation. J. Chem. Phys. 2004, 121, 8261-8270.
9. Gianturco, F. A.; Paesani, F., The He–OCS van der Waals potential from model
calculations: Bound states, stable structures, and vibrational couplings. J. Chem. Phys. 2000, 113,
3011-3019.
45
10. Paesani, F.; Whaley, K. B., Interaction potentials and rovibrational spectroscopy of HeN-
OCS complexes. J. Chem. Phys. 2004, 121, 4180-4192.
11. Kwon, Y.; Birgitta Whaley, K., Atomic-Scale Quantum Solvation Structure in Superfluid
Helium-4 Clusters. Phys. Rev. Lett. 1999, 83, 4108-4111.
12. Roy, P.-N., Microscopic molecular superfluid response: Theory and simulations. Rep.
Prog. Phys. 2014, 77, 046601.
13. Lemeshko, M., Quasiparticle approach to molecules interacting with quantum solvents.
Phys. Rev. Lett. 2017, 118, 095301.
14. Coccia, E.; Bodo, E.; Marinetti, F.; Gianturco, F. A.; Yildrim, E.; Yurtsever, M.; Yurtsever,
E., Bosonic helium droplets with cationic impurities: Onset of electrostriction and snowball effects
from quantum calculations. J. Chem. Phys. 2007, 126, 124319.
15. Crofton, M. W.; Jagod, M. F.; Rehfuss, B. D.; Kreiner, W. A.; Oka, T., Infrared-
Spectroscopy of Carbo-Ions .III. ν3 Band of Methyl Cation CH3
+
. J. Chem. Phys. 1988, 88, 666-
678.
16. Olkhov, R. V.; Nizkorodov, S. A.; Dopfer, O., Intermolecular interaction in the CH 3
+
–He
ionic complex revealed by ab initio calculations and infrared photodissociation spectroscopy. J.
Chem. Phys. 1999, 110, 9527-9535.
17. Liu, X.; Gross, R. L.; Suits, A., "Heavy Electron" Photoelectron Spectroscopy:
Rotationally Resolved Ion Pair Imaging of CH3
+
. Science 2001, 294, 2527 - 2529.
18. Herzberg, G., Molecular Spectra and Molecular Structure II. Infrared and Raman Spectra
of Polyatomic Molecules. D. Van Nostrand Company, Inc.: Princeton, New Jersey, 1956.
19. Hoshina, H.; Skvortsov, D.; Sartakov, B. G.; Vilesov, A. F., Rotation of methane and silane
molecules in He droplets. J. Chem. Phys. 2010, 132, 074302.
20. Oka, T., Nuclear spin selection rules in chemical reactions by angular momentum algebra.
J. Mol. Spectrosc. 2004, 228, 635-639.
21. Slipchenko, M. N.; Vilesov, A. F., Spectra of NH3 in He droplets in the 3 μm range. Chem.
Phys. Lett. 2005, 412, 176-183.
46
22. Töpfer, M.; Salomon, T.; Kohguchi, H.; Dopfer, O.; Yamada, K. M. T.; Schlemmer, S.;
Asvany, O., Double Resonance Rotational Spectroscopy of Weakly Bound Ionic Complexes: The
Case of Floppy CH3
+
-He. Phys. Rev. Lett. 2018, 121, 143001.
23. Dopfer, O., Spectroscopic and theoretical studies of CH3
+
-Rgn clusters (Rg = He, Ne, Ar):
from weak intermolecular forces to chemical reaction mechanisms. Int Rev Phys Chem 2003, 22,
437-495.
24. Topfer, M.; Schmid, P. C.; Kohguchi, H.; Yamada, K. M. T.; Schlemmer, S.; Asvany, O.,
Infrared photodissociation of cold CH3
+
-He2 complexes. Mol. Phys. 2019, 117, 1481-1485.
25. Verma, D.; Erukala, S.; Vilesov, A., Infrared Spectroscopy of Water and Zundel cations in
Helium Nanodroplets. J. Phys. Chem. A 2020, 124, 6207-6213.
26. Hartmann, M.; Miller, R. E.; Toennies, J. P.; Vilesov, A., Rotationally resolved
spectroscopy of SF6 in liquid-helium clusters - A molecular probe of cluster temperature. Phys.
Rev. Lett. 1995, 75, 1566-1569.
27. Gomez, L. F.; Sliter, R.; Skvortsov, D.; Hoshina, H.; Douberly, G. E.; Vilesov, A. F.,
Infrared Spectra in the 3 μm Region of Ethane and Ethane Clusters in Helium Droplets. J. Phys.
Chem. A 2013, 117, 13648-13653.
28. Callegari, C.; Conjusteau, A.; Reinhard, I.; Lehmann, K. K.; Scoles, G.; Dalfovo, F.,
Superfluid Hydrodynamic Model for the Enhanced Moments of Inertia of Molecules in Liquid
4
He. Phys. Rev. Lett. 1999, 83, 5058-5061.
29. Kuyanov, K. E.; Slipchenko, M. N.; Vilesov, A. F., Spectra of the ν1 and ν3 Bands of Water
Molecules in Helium Droplets. Chem. Phys. Lett. 2006, 427, 5-9.
30. Nauta, K.; Miller, R. E., Metastable vibrationally excited HF (v=1) in helium nanodroplets.
J. Chem. Phys. 2000, 113, 9466-9469.
47
Chapter 4. Infrared Spectroscopy of Carbo-Cations upon Electron Ionization
of Ethylene in Helium Nanodroplets
This chapter is based on publication by S.ERUKALA, A.SINGH, A.FEINBERG AND
A.F.VILESOV, “Infrared Spectroscopy of Carbo-Cations upon Electron Ionization of Ethylene in
Helium Nanodroplets”, J.Chem.Phys., 155, 084306 (2021)
Electron impact ionization of helium droplets doped with ethylene molecules and clusters
yields diverse CXHY
+
cations embedded in the droplets. The ionization primarily produces C2H2
+
,
C2H3
+
, C2H4
+
and CH2
+
, whereas larger carbocations are produced upon reactions of the primary
ions with ethylene molecules. Vibrational excitation of the cations leads to the release of bare
cations and cations with a few helium atoms attached. Laser excitation spectra of the embedded
cations show well resolved vibrational bands with a few wavenumber widths - an order of
magnitude less than previously obtained in solid matrices or molecular beams by tagging
techniques. Comparison with previous studies of free and tagged CH2
+
, CH3
+
, C2H2
+
, C2H3
+
and
C2H4
+
cations shows that the helium matrix typically introduces a shift in vibrational frequencies
of less than about 20 cm
-1
, enabling direct comparisons with the results of quantum chemical
calculations for structure determination. This work demonstrates a facile technique for production
and spectroscopic study of diverse carbocations, which act as important intermediates in gas and
condensed phases.
48
4.1 Introduction
Molecular cations are important intermediates in the chemistry of condensed phases or in
the gas phase such as in earth’s upper atmosphere or outer space.
1-4
In comparison to neutrals,
cations remain much less studied. Information on their structure mainly come from NMR studies.
5
The structure of many rather simple cations such as CH4
+ 6, 7
, CH5
+ 8-10
, C4H7
+ 11-13
etc. is still
debated. Cation structure can be implied from characteristic vibrational frequencies and infrared
intensities upon comparison with theoretical results from quantum chemical calculations.
14-16
Because of the low density, only absorption spectra of small cations, such as CH2
+
,
17
CH3
+
,
18
C2H2
+
,
19
C2H3
+
20
etc. were obtained in the gas phase. The study of larger cations usually involves
action spectroscopy in molecular beams. The ions of interest are tagged with Ar atoms or hydrogen
molecules while their absorption is tracked by dissociation of the complexes.
21-24
Tagging
methods, however, may cause the complexes to exhibit band splitting due to lowering of symmetry
or the presence of isomers. The interpretation of the spectra of the complexes can also require
extensive quantum chemical calculations. Recently introduced cooling techniques in cryogenic
traps filled with helium gas has enabled lowering the temperature of the cations to ~ 5-10 K and
tagging with the most weakly bound helium atoms.
21, 25, 26
Experiments in cryogenic traps were
also used to obtain the spectra of free (untagged) ions such as CH5
+ 10
and C3H2
+ 27
by monitoring
faster reactivity of vibrationally excited ions.
Isolation in solid matrices composed of argon, neon and H2 presents another opportunity
to ion spectroscopy.
28-32
Although solid matrices have been used extensively for absorption
spectroscopy of molecules and radicals, their use for ions is more involved due to the small
concentration of ions, large background signal from neutral precursors, and lack of mass
resolution. Isolation in liquid helium droplets is a natural extension of this approach. Liquid helium
49
is the ultimate spectroscopic matrix for a variety of reasons: its low temperature of ~ 0.4 K,
superfluid state and related homogeneous environment, weak interaction with dopants, and high
ionization potential (24.6 eV). For about 3 decades, spectroscopy in helium droplets has been used
extensively to study molecules and molecular clusters.
33-37
However, ion spectroscopy made its
mark in the last decade after Drabbels et al.
38
showed that that ions could be produced in helium
droplets via photoionization. Another important experiment was that of von Helden et al.
39
who
showed that ions originating from an electrospray could be captured by helium droplets. It was
also obtained that the embedded ions could be set free upon irradiation with resonant infrared
radiation, providing a convenient monitor of the absorption.
38,40
Nevertheless, employing
photoionization or electrospray techniques require a more involved apparatus compared to a
typical helium droplet spectroscopic setup and actually imposes limitations on the kind of ions that
can be studied. Electron impact ionization presents a viable alternative. It’s well known that
electron impact ionization of helium droplets doped with neutral species leads to the production of
molecular ions and ionic fragments.
41, 42
A few percent of those produced can be decorated with
several helium atoms, enabling a variant of the tagging experiments. For example, Scheier et al.,
showed that the electron impact ionization of C60 in helium droplets leads to the formation of
HenC60
+
clusters with n ≲ 100.
43
A. Ellis et al. recently produced complexes of protonated acetic
acid and protonated carbon dioxide tagged with a few helium atoms
44,45
and reported their infrared
spectra. We have shown that embedded H2O
+
, H5O2
+
and CH3
+
ions solvated in droplets of few
thousand helium atoms can be obtained upon electron impact ionization of embedded H 2O and
CH4 precursors, respectively.
46, 47
In this work, we show that a variety of embedded CXHY
+
cations can be produced upon
electron impact ionization of ethylene-doped helium droplets. Absorption of multiple infrared
50
photons leads to release of bare cations as well as their clusters with a few attached helium atoms.
Infrared spectra of the cations in the CH-stretching range show narrow vibrational bands with a
few wavenumber widths, which is an order of magnitude less than previously obtained in solid
matrices or in tagged complexes. Comparison with previous studies of some small free cations
shows that the helium matrix introduces a shift in vibrational frequencies of about or less than ±20
cm
-1
, which enables direct comparisons with the results of quantum chemical calculations. This
work demonstrates the utility of a straightforward electron impact ionization technique for
production and spectroscopic study of carbocations embedded in helium droplets which could be
expanded using the appropriate neutral precursors.
4.2 Experimental
Helium droplet pulses of ~ 250 μs width are produced upon expansion of helium gas at
stagnation pressure of P0 = 20 bar and temperature of T0 = 23 K in vacuum through a 1 mm
diameter pulsed nozzle (General Valve series 99) attached to a Sumitomo RDK 408 refrigerator.
48
At these conditions, the droplets have average size of ~7000 He atoms.
46
Upon collimation by a 2
mm diameter skimmer, He droplets enter the 44 cm long pickup chamber where they capture
ethylene molecules. Ethylene backfills the entire vacuum chamber. Its pressure is regulated by a
leak valve and measured by an ionization pressure gauge. Further downstream, doped droplets
pass through a differential pumping stage and enter the detection chamber that hosts a quadrupole
mass spectrometer (QMS) (Extrel MAX 500) with an additional axial external ionizer placed ~ 20
cm upstream from the ionizer of the QMS, which will be referred to as probe ionizer. In standard
QMS operation, doped droplets are ionized by electron impact in the probe ionizer. During the
spectroscopic experiments reported in this work the droplets are ionized with the external ionizer
51
set to 100 eV, 10 mA. Upon ionization, heavy droplet-ion moieties continue traversing towards
the ion range of the probe ionizer, whereas light moieties are rejected by Einzel lenses which acts
as a high pass filter.
Doped ionic droplets are irradiated by a focused infrared laser beam when they pass
through the ion range of the probe ionizer. Absorption of several infrared quanta leads to
production of free ions, which are then extracted, mass selected by the QMS and detected by an
MCP.
46
The signal from the QMS is amplified and measured by a SR250 boxcar integrator with
an appropriate gate and delay with respect to the laser pulse to account for the time of flight of the
ions through the QMS. For measurements of the IR spectra with QMS fixed to a particular mass
the gate was set to ~ 10 μs. This work employed an unseeded pulsed optical parametric oscillator-
amplifier (Laser Vision, spectral resolution: ~ 1 cm
-1
, pulse duration ~ 7 ns, pulse energy
~ 5 - 8 mJ, repetition rate 20 Hz). The absolute frequency of the laser is calibrated using the photo-
acoustic spectrum of methane and ammonia molecules.
52
4.3 Results
Figure 4.1. Mass spectra obtained with the probe ionizer. Black trace - baseline due to residual gas in the
detection chamber. Red trace - neat helium droplets. Blue trace - helium droplets dopped at 4.5×10
-6
mbar
of ethylene. The traces are plotted to the same scale.
Figure 4.1 shows the mass spectra of the helium droplets upon standard ionization in the
probe ionizer. The intensity was recorded around the maxima of the droplet pulses within the gate
of 150 μs. The gate was delayed by ~3.07 ms with respect to the trigger of the pulsed valve to
account for the time of flight of the droplets in the vacuum apparatus. The black trace shows the
base signal with the gate valve (between the pumping stage and the detection chamber) closed and
a residual pressure in the detection chamber of ~ 5×10
-9
mbar. The peaks correspond to the H
+
,
H2
+
H2O
+
, N2
+
and CO2
+
ions and some other weaker peaks from residual gases. The red trace
shows the mass spectrum upon ionization of the neat helium droplet beam. It contains a sequence
of HeN
+
peaks that exhibit decreasing intensity with N.
41, 49
It is seen that with the helium droplet
53
beam, the intensity of the residual gas peaks of H2O
+
and OH
+
increases. We assign this effect to
ionization of water molecules in the residual gas by He* and He
+
, which are produced upon the
electron impact. Peaks due to N2
+
and CO2
+
overlap with He7
+
and He11
+
, respectively. Due to low
residual gas pressure along the helium droplet beam path of ~10
-9
mbar, this additional signal at
masses of OH
+
, H2O
+
, N2
+
and CO2
+
cannot stem from the impurities captured by the droplets.
Blue trace shows the mass spectrum with 4.5 ×10
-6
mbar of ethylene in the pickup chamber. New
intense peaks in the spectrum correspond to C2H2
+
, C2H3
+
and C2H5
+
. The comparison of red and
blue traces also shows that the ionization of the doped droplets yields CH2
+
, C2H4
+
, C3H3
+
, C3H5
+
,
C4H5
+
, C4H7
+
as well as some other less abundant ions.
Figure 4.2. Ethylene pickup pressure dependence of the signal at M=26, M=27 and M=28 is shown by
black squares. In c) the baseline signal due to He 7
+
and N 2
+
at zero ethylene pressure was subtracted. Red
curves are fits with Poison dependence for pickup of single molecule
Figure 4.2 shows the dependence of the intensity of the C2H2
+
(a) C2H3
+
(b) and C2H4
+
(c)
peaks in the mass spectrum versus ethylene pressure, P, in the pickup chamber. Here (and later)
the given pressure corresponds to the ion gauge reading. The absolute pressure could be obtained
by dividing the reading by the sensitivity coefficient for ethylene of 2.3. The curves are fits of the
data points by Poisson probability: 𝐼 𝑘 (𝑃 ) = 𝐶 ∙
(𝑃 /𝐴 )
𝑘 𝑘 !
𝑒 −𝑃 /𝐴 (1)
54
for capture of k- ethylene molecules per droplet, with A and C fitting parameters. At P = A, on
average one molecule is captured per droplet with I1(P=A) being the maximum. It is seen that fits
with k=1 provide a good representation of the data points. At small pressure, the intensity rises
linearly, consistent with pickup of single molecules. For both C2H2
+
and C2H3
+
, the maxima at 2.7
×10
-6
mbar corresponds to the pickup pressure at which the droplets are doped with a single
ethylene molecule on average. This indicates that the free C2H2
+
and C2H3
+
cations are
predominantly produced upon the ionization of droplets containing single ethylene molecules. An
overshoot of the data points at P > 10
-5
mbar indicates that some minor yield of the ions are from
ethylene clusters. In comparison, the dependence for C2H4
+
has maximum at 5.6×10
-6
mbar, which
corresponds approximately to two captured ethylene molecules per droplet. Nevertheless, the
initial rise is still linear and the Poisson dependence with k=1 gives a good fit. This discrepancy
most likely indicates that the C2H4
+
ions are produced from single embedded ethylene molecules
as well as from small clusters, such as dimers and trimers.
Figure 4.3. The total yield of ions upon laser irradiation of the ethylene doped ionized droplets. The insert
shows an additional scan in the indicated range recorded with about a factor of ~10 higher amplification.
The assignment of the spectral peaks is indicated.
55
Figure 4.3 shows the spectrum of the total ion yield upon laser irradiation of droplets doped
with ethylene (4×10
-6
mbar) and that were ionized in the external ionizer with the probe ionizer
off. During the measurements, the DC of the QMS was off so that all ions produced upon laser
irradiation were guided to the detector independent of their mass. The boxcar gate was set to 150
μs to accept ions with different masses which have different time of flight through the QMS. The
main trace was obtained at low amplification to avoid saturation of the most intense peaks. The
trace in the insert was recorded separately with a factor of ~10 larger amplification to discern weak
peaks. Figure 4 shows that the full ion spectrum has large number of peaks assigned to different
ions as it will be explained in the following. It is seen that the most intense peaks correspond to
C2H2
+
and C2H3
+
ions which are also most prominent in the mass spectrum in Fig. 4.1.
Figure 4.4. Mass spectra measured with laser parked at the spectral peaks as indicated in the legend. Trace
a) was obtained with laser blocked.
56
For the assignment of the peaks in Fig. 4.3, the laser frequency was set to the maximum of
a spectral peak and the mass spectrum was recorded with the boxcar gate of 150 μs. Fig. 5 (b-f)
shows examples for CH2
+
, C2H2
+
, C2H3
+
, C2H4
+
and C3H5
+
as indicated in the legend. The
corresponding lines are marked by asterisks in Fig. 4.3. Trace a) shows the baseline mass spectrum
recorded with the laser beam blocked and contains some weak peaks due to H2O
+
, N2
+
, O2
+
and
CO2
+
. The ions are likely produced from the residual gas ionization within the ion range of the
probe ionizer upon encounters with metastable He* atoms which are produced in the external
ionizer. In comparison, traces b)-e) show some new features. Trace b) recorded upon laser
excitation at 3159.6 cm
-1
has a strong peak at M=14 au, and a sequence of peaks at higher masses
with increments of 4 au and decreasing intensity. These peaks are assigned to free CH 2
+
ions and
CH2
+
HeN clusters. Thus, the spectral peak at 3159.6 cm
-1
belongs to CH2
+
in helium droplets. In
the mass spectra, the lowest mass of the laser induced peak gives the parent ion in helium droplets.
Accordingly traces c)-e) correspond to the excitation of C2H2
+
, C2H4
+
, C3H5
+
and C2H3
+
ions in
helium droplets, respectively. Similarly, the other spectral peaks in Fig. 4.3 were assigned to
different ions. The appearance of the M
+
HeN peaks depends on the type of ion and the propensity
towards the release of the M
+
HeN clusters which seems to be weaker for larger ions, so that the
C3H5
+
HeN peaks are not discernable. Trace c) is unusual in that besides M=26 which is due to
C2H2
+
ions, it contains a sizable additional peak at M=28 and some weak progression of the
corresponding helium cluster peaks. It is feasible that M=28 signal comes from weakly bound
complexes C2H2
+
with H2 molecules in helium droplets which has close frequency with C2H2
+
.
57
Figure 4.5. Spectra recorded at M=14 (pink), 26 (blue), 27 (red) and 28 (black).
Figure 4.5 shows the infrared spectra measured with the QMS fixed at M=14 (pink), 26
(blue), 27 (red) and 28 (black). No other bands were observed in broader range from 2600 to 3300
cm
-1
. The spectrum of CH2
+
has an intense peak at 3159.6 cm
-1
and a weak peak at 3146.9 cm
-1
which will be further discussed in Section 4.4.5. The prominent peak in the blue trace at 3140.0
cm
-1
is due to acetylene cations, C2H2
+
which has a single C-H infrared active band. The weak
peak in the blue trace at 3159.5 cm
-1
is assigned to CH2
+
in helium droplets that contributes to
measurements at M=26 via CH2
+
-He3 complexes. The two peaks in the black trace at 3007.0 and
3105.3 cm
-1
belong to ethylene cations, C2H4
+
. Similar to ethylene, ethylene cations are expected
to have two infrared active C-H stretching bands.
30,31,50
The higher frequency ν9 band is a b-type
transition due to the in-phase asymmetric stretch of the two CH2 groups. The ν9 band of C2H4
+
appears as a triplet which is assigned to the partially resolved rotational structure of the
perpendicular band, similar to that observed for CH3
+
in helium.
47
The third intense peak at 3140.8
cm
-1
may not be assigned to C2H4
+
which must only have two C-H infrared active bands. The
position of the peak is very close and about 0.8 cm
-1
shifted with respect to the 3140.0 cm
-1
peak
58
of C2H2
+
. Therefore, we tentatively assigned this peak to C2H2
+
-H2 complexes as it is obtained
with QMS tuned to M=28. The spectrum of C2H3
+
(red trace) has a single, strong peak consistent
with bridged structure of the complex.
20,
51
The bands assigned to CH2
+
, C2H2
+
and C2H3
+
in Fig.
4.5 have frequencies close to the corresponding bands of free ions.
17, 19, 20, 52
Figure 4.6. Laser pulse energy dependence of the signal for C 2H 2
+
at 3140.0 cm
-1
(blue), C 2H 3
+
at 3145.7
cm
-1
(red) and C 2H 4
+
at 3105.3 cm
-1
(black).
Figure 4.6 shows the laser pulse energy dependence of the C2H2
+
, C2H3
+
and C2H4
+
bands
in Fig. 4.5. The laser pulsed energy is measured at the entrance to the vacuum apparatus. For the
shown bands, the signal is very low at the laser pulse energy of less than ~ 0.5 mJ and then rises
approximately linear at >1 mJ. Similar laser pulse energy dependencies with an onset threshold
were observed previously for H5O2
+ 46
and CH3
+ 47
in helium droplets, consistent with the
absorption of multiple photons required for producing free ions. The onset threshold energy is
approximately proportional to the band width divided by infrared intensity of the transition.
46
However the values of the onset in this work and in Ref.
46
should not be compared due to the
59
realignment of the focusing lens between the measurements. The spectra reported in this work
were measured at the maximum available laser pulse energy of ~ 4 mJ/pulse.
Figure 4.7. Ethylene pickup pressure dependences of the laser induced signal for C 2H 2
+
at 3139.8 cm
-1
,
C 2H 3
+
at 3144.4 cm
-1
and C 2H 4
+
at 3105.3 cm
-1
as measured at M = 26 (a), M = 27 (b), and M = 28 (c),
respectively. Curves are fits of the data points by Poisson probability for the capture of k = 1 (red) and k =
2 (blue) molecules per droplet.
Figure 4.7 shows the dependence of the intensity of the C2H2
+
(a), C2H3
+
(b), and C2H4
+
(c) bands vs the ethylene pressure in the pickup chamber. The curves are fits to Poisson
distributions for the capture of k molecules per droplet. It is seen that for all ions the intensities
60
rise linearly at small pressures, however the fits with k = 2 provide better agreement with the
experimental results. It likely indicates that the signal stems from single molecules as well as from
small clusters, whose contributions could not be accurately deter- mined. The maxima of the
dependences for (a) C2H2
+
and (b) C2H3
+
in Fig. 4.7 are at 3.0 × 10
-6
mbar and 3.3 × 10
-6
mbar,
respectively. On the other hand, the maximum for k = 2 based on the dependences in Figs. 4.2(a)
and 4.2(b) is expected at about 5.4× 10
-6
mbar. This discrepancy may indicate that the average size
of the ionized droplets is larger than of the neutral ones. This effect may reflect the rejection of the
smaller droplets during the transport from external ionizer to the probe ionizer ion region. Note
that using the Poisson probability function is an approximation because of the droplet size
distribution, changing droplet cross section, and scattering of the beam upon the pickup of the
dopants.
41
In this work, the scattering effect will be amplified by the deflection of charged droplets,
which occurs due to the electric field between the external ionizer and the ion range of the probe
ionizer.
The spectroscopic experiment on ions in helium droplets could be further improved.
Results such as in Fig. 4.3 call for replacing the quadrupole mass spectrometer as used in this work
by a time-of-flight mass spectrometer. This will enable simultaneous measurements of the spectra
for all the embedded ions in a single laser scan. The nonlinear energy dependence of the laser pulse
observed in this work and previously in Refs.
40, 46
is inconvenient and may complicate the
identification of weak bands. This effect relates to the absorption of multiple infrared photons
required for evaporation of the entire droplet. Possible improvement of the technique may involve
production of ions in larger droplets followed by subsequent moderation of the droplet size, which
could be achieved upon multiple collisions with helium atoms at room temperature in an RF ion
guide, as recently demonstrated by Scheier et al.
53
61
4.4 Discussion
This work confirms earlier observations
46, 47
that electron impact ionization of helium
droplets doped with molecules leads to the production of both free as well as embedded ions.
Doping with ethylene molecules yields a variety of CXHY
+
cations. The most abundant free and
embedded fragments are splitter ions such as CH2
+
, C2H2
+
, C2H3
+
and C2H4
+
Observation of
weaker signals from ions such as C3H5
+
and C4H5
+
shows that ionization of ethylene clusters can
yield larger embedded cations containing three or four carbon atoms resulting from ion-molecule
reactions. Resonant laser excitation of the embedded species causes the release of free cations
which may have several attached helium atoms. Infrared spectra are obtained by monitoring the
intensity of specific released ions. All observed C-H bands for different cations appear as narrow
peaks with few wavenumber widths.
4.4.1 Formation of ions by charge transfer from He
+
The Poisson pressure dependences in Fig. 4.2 indicate that free C2H2
+
and C2H3
+
are mainly
produced upon ionization of single embedded ethylene molecules. At small pickup pressures when
doped droplets mostly contain single ethylene molecules, the dominant cationic products in the
probe ionizer, such as in Fig. 2.5, are C2H2
+
and C2H3
+
with yield ratio of about 1:3. Electron
impact ionization of free ethylene molecules yields C2H4
+
, C2H3
+
, C2H2
+
, and C2H
+
ions with the
intensity in the ratio of ~ 100:65:55:10.
54
This is inconsistent with observations in helium droplets.
Although the electron impact ionization cross section of ethylene is a factor of 10 larger than for
helium atoms, the probability of ionizing one of the 7000 helium atoms of the droplet is about a
factor of 700 larger than for ethylene molecule. Therefore, cations are likely produced in charge
transfer reactions from He
+
ions. Charge transfer from He
+
to C2H4 in the gas phase produces
C2H4
+
, C2H3
+
, C2H2
+
, C2H
+
, CH3
+
and CH2
+
ions with a yield ratio of ~ 7:5:64:13:2:12.
55
Although
62
the reaction with He
+
is consistent with the observed high yield of C2H2
+
in Fig. 4.1, it also predicts
a small yield of C2H3
+
which is the dominant ion in Fig. 4.1. It is likely that the yields of ions
produced in charge transfer in helium droplets differs from those in gas. In the droplets, He
+
ions
rapidly form He3
+
units, which may induce a different fragmentation pattern as compared with
bare ions. Interaction with the helium environment may result in some relaxation of the highly
excited ionic states from which the dissociation takes place. However, the study of these effects is
beyond the scope of the present study. For now, we assume that C2H2
+
, C2H3
+
, C2H4
+
and CH2
+
are the primary products of the ionization of single ethylene molecules in helium droplets in
reactions (2-5).
C2H4 + He
+
→ C2H2
+
+ H2 + He (2)
C2H4 + He
+
→ C2H3
+
+ H + He (3)
C2H4 + He
+
→ C2H4
+
+ He (4)
C2H4 + He
+
→ CH2
+
+ CH2 + He (5)
The infrared spectrum in Fig. 4.3 indicates that C2H2
+
and C2H3
+
and to smaller extent,
C2H4
+
and CH2
+
are the dominant embedded ions. The pickup pressure dependence indicates that
the primary ions are produced either from single embedded molecules or from dimers . The relative
efficiency of producing free and embedded ions may be gleaned upon by the comparison of the
QMS signal ion intensity as obtained without a laser and with a laser in measurements described
in relation to Fig. 4.1 and Fig. 4.5, respectively. We will refer to these intensities as Iprobe and Ilaser,
respectively. Both Iprobe and Ilaser can be expressed as integrals over time. Ilaser is the integral over
the signal duration of ~5 μs in width. This time adds from the extraction time of the ions from the
ion range of the probe ionizer of ~3 μs as well the effect of the time constant of the pre-amplifier.
We assume that the ions released from the droplets inside the laser beam waist along the axis of
63
the ion range of ~1 cm length, see Fig 2.5. Taking the velocity of the droplet beam of ~400 m/s,
the flight time of the droplets through the ion range is ~25 μs and was used for the integration to
obtain Iprobe. We obtain that for both dominant C2H2
+
and C2H3
+
ions, Iprobe/Ilaser ≈ 50. For further
comparison we note that Iprobe is drawn upon ionization of the entire helium droplet beam whose
cross section is given by the 5 mm diameter aperture of the ion range. In comparison, Ilaser stems
from a laser beam waist of ~1 mm diameter as estimated from paper burns, i.e. a factor of ~25
smaller volume. The laser pulse energy dependence in Fig. 4.6 does not show any saturation, which
indicates that the maximum laser pulsed energy used of ~ 4 mJ is insufficient for the release of
ions from all droplets within the laser beam waist. In addition, the measurements of Ilaser were
performed at a fixed mass of the dominant bare ion products and did not include contribution from
the clusters of ions with a few helium atoms, such as in Fig. 4.4. Therefore, we conclude that the
electron impact ionization of the ethylene doped droplets with about 7000 helium atoms have
comparable yield of free and embedded C2H2
+
and C2H3
+
ions. In fact, the yield of the embedded
products may turn out to be dominant considering that Ilaser considerably underestimates the
amount of embedded ions.
It has long been observed that the electron impact ionization of helium droplets yields a
series of HeN
+
ions as well as ions that stem from the embedded molecules and clusters.
41, 42
The
electron impact ionization of one of the helium atoms in the droplet is followed by charge transfer
to the embedded molecule and dissociative ionization. Due to the larger ionization potential of
helium (IPHe=24.6 eV) as compared with other molecules (IPethylene= 10.5 eV) the resulting energy
release of about 14 eV is sufficient for the evaporation of the entire droplet. Penning ionization
including He* (19.8 eV) metastable atoms which are also produced upon electron impact may
constitute another ionization channel. Recall that 1 eV of energy is sufficient for thermal
64
evaporation of ~1500 helium atoms from the droplet. Therefore, it was widely believed that
droplets of few thousands of helium atoms are destroyed upon electron impact giving rise to
predominantly free dopant ions and small HeN
+
splitter ions.
42,56
Our results disagree with this
conjecture and show a high yield of embedded ions. The precise mechanism for the mitigation of
energy released upon charge transfer remains to be studied. It is feasible that a large portion of this
energy is transferred to the translational energy of the products that leave the droplet. Hydrogen
atoms are known to repel helium atoms and H2 molecules are weakly bound and are likely easily
escaping the droplets. Small neutrals such as CH2, C2H2 and C2H4 may also leave the droplet if
they possess sufficient kinetic energy. Previously, it was observed that free alkyl radicals leave
droplets upon photodissociation of embedded alkyl iodides such as CF3I, C2H3I and CH3I.
57
4.4.2. Formation of larger ions by ion molecule reactions
Larger ions containing three or four carbon atoms can be produced from primary ions via
the following ion molecule reactions:
55
C2H2
+
+ C2H4 → C2H4
+
+ C2H2 (6)
C2H2
+
+ C2H4 → C3H3
+
+ CH3 (7)
C2H2
+
+ C2H4 → C4H5
+
+ H (8)
C2H3
+
+ C2H4 → C2H5
+
+ C2H2 (9)
C2H3
+
+ C2H4 → C3H3
+
+ CH4 (10)
C2H3
+
+ C2H4 → C4H5
+
+ H2 (11)
C2H4
+
+ C2H4 → C3H5
+
+ CH3 (12)
C2H4
+
+ C2H4 → C4H7
+
+ H (13)
65
The pressure dependence of the intensity of cations with three and four carbon atoms have
a quadratic initial pressure rise and maxima around 6 ×10
-6
mbar, consistent with their production
in the bimolecular charge transfer reactions (6-13). Reactions (6-13) were identified in the gas
phase at room temperature. It is unclear how the rate of these reactions would change at
temperature of about 1 K. On the other hand, the ions will likely originate in some vibrationally or
electronic excited states and may react before cooling. We did not observe any indication of the
presence of unreacted cationic complexes with ethylene molecules in the infrared spectra in Fig.
4.3. In comparison, previous studies of the embedded molecules inevitably revealed additional
bands due to small molecular clusters. These findings may indicate that the ion molecular reactions
proceed to completion in helium droplets such that no unreactive complexes of primary ions with
ethylene molecules remain. In addition to the bimolecular reactions above, C4H7
+
can also be
produced in droplets via recombination reaction:
C2H3
+
+ C2H4 → C4H7
+
(14)
The reaction schemes presented above explain the presence of all ions identified upon
electron impact ionization, both free and embedded. Accordingly, CH2
+
, C2H2
+
, C2H3
+
and C2H4
+
are produced upon charge transfer from He
+
ions in reactions (2-5) and are the most abundant
products. C3H3
+
, C3H5
+
, C4H5
+
and C4H7
+
are secondary ions with smaller abundance originated
from the reaction of primary ions with ethylene molecules (6-14). Therefore, the composition of
the embedded ions produced from some other precursors may be anticipated knowing the primary
ions produced upon the electron impact ionization and reactions of primary ions with the
precursors. The only notable exception from this scheme is C2H5
+
which is expected to be
produced by proton transfer in reaction (9). Free C2H5
+
ions produce a strong peak in Fig. 4.1. On
the other hand, the absence of the signal assigned to these ions in Fig. 4.3 indicates their retention
66
by helium droplets is inefficient. The lack of protonated ethylene contrasts with our previous
studies of ionization of water clusters in helium droplets
46
, where a strong spectrum from
protonated water dimers (H5O2
+
) was observed. The origin of the inefficient production of
embedded protonated hydrocarbons deserves further investigation.
4.4.3. Free ions
Figure 4.5 shows that substantial fraction of free ions produced upon infrared laser
excitation has one or few attached helium atoms. For CH2
+
, the signals due to bare ions and ions
containing a few helium atoms are comparable, whereas for C2H2
+
and C2H4
+
the peak due to bare
ions is a factor of ~10 stronger. A similar effect was found by Drabbels et al., for aniline ions
released from helium droplets upon infrared excitation.
38
The binding energy of helium atoms to
a molecular ion such as CH3
+
is of the order of 100 - 200 cm
-1
for the most strongly bound helium
atoms.
58
Our results are consistent with free ions released upon multiple sequential events of
absorption of infrared quanta by cations followed by the transfer of the absorbed energy to the
droplets and evaporation of helium atoms.
46
Absorption of one quantum of ~3000 cm
-1
radiation
leads to thermal evaporation of about 600 helium atoms. Thus, the absorption of n≈10 quanta is
required for the thermal evaporation of the droplets used in this work. Upon absorption of the first
n-1 quanta, the helium droplet size decreases such that it contains a few hundred atoms. The
vibrational frequency of the molecules within a fraction of the wavenumber does not depend on
the number of helium atoms in droplets larger than ~500 atoms.
59
Upon absorption of the last n-th
photon, the last few hundred helium atoms evaporate leaving bare cations or those having a few
helium atoms attached, depending on the number of helium atoms in the droplet before the last
absorption. Bare cations or those with a few helium atoms attached have their vibrational
frequency shifted by few wavenumbers with respect to those in droplets. Therefore, they come out
67
of the resonance with the laser infrared radiation and cannot be further excited. The resulting
abundance of the ions with a few helium atoms attached may depend on several factors, such as
binding strength of helium atoms with cations as well as the line width of the absorption and the
amount of spectral shift induced by the helium matrix. Nevertheless, the appearance of ions with
helium atoms attached indicates that the cations have internal temperature of about or less than
100 K. Therefore, the measurements could likely be expanded to complexes of ions with other
species, such as C2H2
+
-H2, which presence is suggested in Fig. 4.4.
4.4.4 Spectroscopy and comparison with gas, matrix and tagging
Table 4.1 shows a comparison of the frequencies of the C-H bands in small carbocations
studied in this work with previous measurements in gas phase, by tagging with rare gas (Rg) atoms
and in solid matrices. It’s seen that frequencies in helium are within about ±20 cm
-1
and coincide
with those in the gas phase when available. The gas phase absorption spectra of small cations listed
in Table 4.1 were obtained in discharges, which could not be easily extended to larger ions due to
small number densities. In some cases, the vibrational frequencies of cations were deduced from
the high-resolution photoelectron spectra (PES), such as for C2H4
+
.
50
However, the PES has
different selection rules and not all infrared active vibrations could be accessed. Tagging of cations
by Rg atoms was used to obtain the spectra of numerous cations.
22, 23, 62, 63
In this technique, the
spectrum of a complex of a cation with a Rg atom is obtained which often has some extra bands
which requires some additional efforts to interpret. The bands of the complexes typically have
widths of the order of few tens of the wavenumbers, which often causes the spectral congestion of
different bands. Figure 4.8 shows a comparison of the spectrum of the C2H3
+
in helium droplets
with that for the argon-tagged cations.
51
It is seen that the spectrum in helium is a factor of ~10
narrower as compared with the tagged cations where the linewidth was ascribed to unresolved
68
rotational structure of the complexes with internal temperature of ~100 K. The spectrum of the
C2H3
+
in helium is represented by a single narrow line, which is consistent with the bridged C2v
non-classical structure in agreement with earlier works.
20, 51
in gas Tagged in matrix in He Neutralized
from Ref.
60
CH2
+
(v3) 3131.37
17
3159.6
a)
3190
CH3
+
(v3) 3107.85
18
Oka
3121.3
+2He
26
3128.1 3160.8
C2H2
+
(v3) 3135.98
19
3152.73
+Ar
61
3140.0 3294.9 FR
3281.9 FR
C2H3
+
(v6)
bridged
3142.3
20
3146.3
+Ar
51
3145.7 2901.9
vinyl
C2H4
+
(v11) 2978.7
from PES
50
3014.4 in Ne
30
3028.7 in Ar
31
3007.0 2988.7
C2H4
+
(v9) 3153.5 in Ar
31
3102.7 3105.5
a) The frequency of the dominant peak in the spectrum.
Table 4.2. C-H vibrational frequencies for cations obtained in gas, via tagging, in solid matrix and in helium
droplets. The last column given the frequencies for the corresponding neutral species. All values and in
units of wavenumbers.
69
Figure 4.8. The comparison of the spectrum of C 2H 3
+
-Ar with the spectrum of C 2H 3
+
in helium droplets.
The vibrational frequencies of neutral molecules in solid matrices and liquid helium
droplets are often shifted towards low frequencies due to larger solvation energy of the
vibrationally excited species.
34-36
Table 4.1 shows that the studied cations in helium, except C2H3
+
,
experience a high frequency shift. This effect may be related to small transfer of the electron
density from the surrounding helium atoms, as it was previously discussed in relation to CH3
+
.
47,
58
Overall the helium matrix shift is of the order or smaller than ±20 cm
-1
, which is comparable
(or better) than the accuracy of quantum chemical calculations currently. Therefore, the
measurement of the frequencies of cations in helium droplets could be compared with the results
of quantum chemical calculations for free cations to determine structure.
4.4.5 Rotation of ions in helium droplets
We previously observed that the v3 band of CH3
+
in helium droplets has three prominent
peaks that were assigned to partially resolved rotational structure of the K'←K" sub-bands of the
perpendicular transition.
47
The K"=0 and K"=1 remain populated in helium droplets due to their
70
different nuclear spin states which interconversion is slow in helium droplets. Similar result was
previously observed for small symmetric molecules, such as SF6, NH3, CH4 and C2H6.
64-67
Previous extensive studies of neutral species in helium droplets have shown that the spectra could
be described by introducing effective rotational constants with values close to or up to a factor of
about three smaller than for free molecules.
34-37
In comparison, the effect was observed to be larger
in CH3
+
and manifested dramatically different for rotation around the C3 axis and perpendicular to
it, where the rotational constants of free molecules of A(1-ζ)=4.05, B=9.27 decrease in helium to
about 3.4 cm
-1
and <0.1 cm
-1
, respectively. This unusually large disparity is explained by a strong
and anisotropic interaction of CH3
+
with helium atoms. The interaction along the partially filled
2pZ orbital of carbon atoms draws two helium atoms to the complex, making an effective He-
CH3
+
-He rotor. On the other hand, we concluded that other atoms in the first solvation shell of
CH3
+
likely continue participating into the exchange with more distant helium atoms of the
superfluid droplet.
Among the ions presented in Fig. 4.5, only the 3153.5 cm
-1
band of C2H4
+
shows the triplet
(3100.3, 3105.3, 3110.4 cm
-1
) which could be assigned to the rotational structure of the b-type ν9
band. The free C2H4
+
in the ground X
2
B3u has an effective D2h symmetry and has nuclear spin
isomers.
68
C2H4
+
is nearly a symmetric top with A=4.77, B=0.93 and C=0.78 cm
-1
.
69
The rotational
constant A= 2.5 cm
-1
and band origin at 3102.7 cm
-1
can be obtained from Fig. 4.5. Taking into
account the width of the sub-bands of δν≈1.5 cm
-1
FWHM, which is very close to nominal laser
linewidth of 1 cm
-1
, we concluded that the B and C constants of C2H4
+
are small (<0.3 cm
-1
) and
could not be determined. This is in agreement with the absence of any rotational structure in the
a-type ν11 3007 cm
-1
band which has δν≈1 cm
-1
. These results show that the A constant for rotation
71
over the C-C axis decreases in helium by about a factor of 2, whereas the decrease of the end over
end rotational constant is probably much larger but could not be determined.
The free C2H2
+
ions have a
2
Πu ground state with rotational constant of B=1.104 cm
-1
and
spin-orbit interaction constant of A=-30.91 cm
-1
.
19
The rotational angular momentum N and the
electron spin angular momentum S couple to make the total angular momentum J=N+S, and the
spectrum consists of two series of F1 and F2 corresponding to J=N+1/2 and J=N-1/2. The Λ-
doubling components have intensity ratio of 3:1 due to the nuclear spin of protons.
19
It is likely
that in helium droplets the ions relax into the lowest spin orbit component as it was previously
observed for NO molecules in helium droplets.
70
Therefore, at low temperature the spectrum
should be represented by the Q(J=3/2) and R(J=3/2) which have splitting of about 5.3 cm
-1
in the
gas phase.
19
The band of C2H2
+
in Fig. 4.5 appears as a single peak with the width of 1.5 cm
-1
FWHM, which indicates that the rotational constant B in helium is much less than in free ions, but
could not be determined in the present work.
Free CH2
+
ion in the ground X
2
A1 state is close to linear with rotational constants
A=69.5 cm
-1
, B=7.85 cm
-1
and C=6.92 cm
-1
.
71
The spectrum of CH2
+
in helium droplets show two
narrow (δν≈1.3 cm
-1
) lines at 3146.8 and 3159.6 cm
-1
with intensity ratio of about 1:10. The
splitting between the lines of 12.8 cm
-1
is rather close to the value of B+C =14.8 cm
-1
in the gas
phase, which hints it may relate to the rotational structure of CH2
+
in helium. The lowest levels of
the para- and ortho- modifications of CH2
+
should be 000 and 101, neglecting electron spin, similar
to that for water molecules.
72
The a-type spectrum of CH2
+
is close to that for a linear molecule
19
and at low temperature should consist of three lines equivalent to the P(1), R(0) and R(1) lines of
a linear rotor. Because only two lines were observed in helium droplets, the assignment could not
be made. If the rotational structure is indeed responsible for the appearance of the two lines of
72
CH2
+
, the position of its band origin in Table 4.1 should be revised. Its current value corresponds
to the frequency of the most intense line in the spectrum.
In the gas phase C2H3
+
is a nearly symmetric top with A=13.3 cm
-1
, B=1.14 cm
-1
and
C=1.05 cm
-1
.
20
The fact that the a-type band of C2H3
+
in helium droplets appears as a single peak
with δν≈1.3 cm
-1
FWHM was observed for in Fig. 4.5 likely indicates considerable drop of the
constants B and C for the end over end rotation in helium. The discussion of the rotational spectra
in this section assumes that the interconversion between the nuclear spin isomers of symmetric
ions is inefficient in helium droplets. This assumption appears valid for CH3
+
and C2H4
+
, however
its generality for other ions should be further evaluated. It is feasible that the relaxation of the
nuclear spin in ions is faster than in neutrals due to magnetic moment induced by rotation of ions
as well as due to magnetic moment of the electron angular momentum and electron spin, such as
in the
2
Π state.
4.5. Conclusions
This work demonstrates that helium droplets are an open system, akin a test tube. It was
well known that neutral precursors are easily captured by the droplets. Here we have shown that
ionization of neutral species produces embedded splitter ions with a high yield. The light products
such as hydrogen atoms and small molecules such as H2, CH2 etc. easily leave the droplets carrying
away excess energy that contributes to stabilization of the cations inside. If ethylene dimers or
clusters are present, the primary ions originate by charge transfer from He
+
ions which react with
ethylene and produce secondary ions. These ions contain three to four carbon atoms that stabilize
by shedding hydrogen atoms and H2 molecules. Ions such as C3H3
+
, C3H5
+
, C4H5
+
, C4H7
+
etc. have
structural isomers interconversion between which was invoked in reactions of carbo-cations.
5
It
73
would be interesting to extend the infrared spectroscopic studies in helium droplets to study of the
structure of larger cations. We expect that larger cations could be produced and studied upon
application of the appropriate precursor molecules. The technique is especially useful for
carbocations which could not be easily produced via other methods such as photoionizatoin
38
or
electrospray
40
.
The vibrational spectra of the cations are obtained upon irradiation with resonant infrared
laser radiation which causes the release of the cations from the droplets. The spectra are nearly
background free and demonstrate a high signal to noise ratio of ~1000 for the strong bands and
concomitant large dynamic range for the measurements. This experiment does not require double
mass spectrometer and/or cryogenic traps that are often applied in the contemporary works with
tagging. Infrared spectra of the cations in the CH-stretching range show narrow vibrational bands
with a few wavenumbers in width, which is an order of magnitude less than previously obtained
in solid matrices or in tagged complexes. Comparison with previous studies of some small free
cations shows that the helium matrix introduces a shift of vibrational frequencies of about ±20 cm
-
1
or less, which enables direct comparisons with the results of quantum chemical calculations for
structure determination. This technique may be especially useful to determine the structure of some
prototypical cations of organic molecules, and radicals as well as protonated species.
4.6. References
1. Agmon, N.; Bakker, H. J.; Campen, R. K.; Henchman, R. H.; Pohl, P.; Roke, S.; Thämer,
M.; Hassanali, A., Protons and Hydroxide Ions in Aqueous Systems. Chem. Rev. 2016, 116, 7642-
7672.
2. Shuman, N. S.; Hunton, D. E.; Viggiano, A. A., Ambient and Modified Atmospheric Ion
Chemistry: From Top to Bottom. Chem. Rev. 2015, 115, 4542-4570.
74
3. Tielens, A. G. G. M., The Physics and Chemistry of the Interstellar Medium. Cambridge
University Press: Cambridge, 2005.
4. Olah, G. A.; Surya Prakash, G. K.; Wade, K.; Molnar, A.; Williams, R. E., Hypercarbon
Chemistry. John Wiley & Sons: Hoboken,New Jersey, 2011.
5. Olah, G. A.; Prakash, G. K. S.; Molnar, A.; Sommer, J., Superacid Chemistry. John Wiley
& Sons: Hoboken, New Jersey, 2009.
6. Frey, R. F.; Davidson, E. R., Potential energy surfaces of CH4
+
. J. Chem. Phys. 1988, 88,
1775-1785.
7. Wörner, H. J.; van der Veen, R.; Merkt, F., Jahn-Teller Effect in the Methane Cation:
Rovibronic Structure and the Geometric Phase. Phys. Rev. Lett. 2006, 97, 173003.
8. White, E. T.; Tang, J.; Oka, T., CH5
+
: The infrared spectrum observed. Science 1999, 284,
135-137.
9. Asvany, O.; Kumar, P.; Redlich, B.; Hegemann, I.; Schlemmer, S.; Marx, D.,
Understanding the infrared spectrum of bare CH5
+
. Science 2005, 309, 1219-1222.
10. Asvany, O.; Yamada, K. M. T.; Brunken, S.; Potapov, A.; Schlemmer, S., Experimental
ground-state combination differences of CH5
+
. Science 2015, 347, 1346-1349.
11. Siehl, H.-U., Chapter One - The Conundrum of the (C4H7)
+
Cation: Bicyclobutonium and
Related Carbocations. In Adv. Phys. Org. Chem., Williams, I. H.; Williams, N. H., Eds. Academic
Press: 2018; Vol. 52, pp 1-47.
12. Olah, G. A.; Reddy, V. P.; Prakash, G. K. S., Long-lived cyclopropylcarbinyl cations.
Chem. Rev. 1992, 92, 69-95.
13. Olah, G. A.; Surya Prakash, G. K.; Rasul, G., Ab Initio/GIAO-CCSD(T) Study of
Structures, Energies, and 13C NMR Chemical Shifts of C4H7+ and C5H9+ Ions: Relative Stability
and Dynamic Aspects of the Cyclopropylcarbinyl vs Bicyclobutonium Ions. J. Am. Chem. Soc.
2008, 130, 9168-9172.
75
14. Buss, V.; Schleyer, P. V. R.; Allen, L. C., The Electronic Structure and Stereochemistry of
Simple Carbonium Ions. In Topics in Stereochemistry, Allinger, N. L.; Eliel, E. L., Eds. John
Wilesy & Sons: 1973; Vol. 7, pp 253-293.
15. Császár, A. G.; Fábri, C.; Sarka, J., Quasistructural molecules. WIREs Computational
Molecular Science 2020, 10, e1432.
16. McCoy, A. B., Diffusion Monte Carlo approaches for investigating the structure and
vibrational spectra of fluxional systems. Int Rev Phys Chem 2006, 25, 77-107.
17. Rosslein, M.; Gabrys, C. M.; Jagod, M. F.; Oka, T., Detection of the Infrared-Spectrum of
CH2
+
. J. Mol. Spectrosc. 1992, 153, 738-740.
18. Crofton, M. W.; Jagod, M. F.; Rehfuss, B. D.; Kreiner, W. A.; Oka, T., Infrared-
Spectroscopy of Carbo-Ions .III. ν3 Band of Methyl Cation CH3
+
. J. Chem. Phys. 1988, 88, 666-
678.
19. Jagod, M. F.; Rösslein, M.; Gabrys, C. M.; Rehfuss, B. D.; Scappini, F.; Crofton, M. W.;
Oka, T., Infrared spectroscopy of carbo‐ions. VI. C–H stretching vibration of the acetylene ion
C2H2
+
and isotopic species. J. Chem. Phys. 1992, 97, 7111-7123.
20. Gabrys, C. M.; Uy, D.; Jagod, M. F.; Oka, T.; Amano, T., Infrared Spectroscopy of
Carboions. 8. Hollow Cathode Spectroscopy of Protonated Acetylene, C2H3
+
. J. Phys. Chem. 1995,
99, 15611-15623.
21. Roithova, J.; Gray, A.; Andris, E.; Jasik, J.; Gerlich, D., Helium Tagging Infrared
Photodissociation Spectroscopy of Reactive Ions. Acc. Chem. Res. 2016, 49, 223-230.
22. Duncan, M. A., Infrared Laser Spectroscopy of Mass-Selected Carbocations. J. Phys.
Chem. A 2012, 116, 11477-11491.
23. Bieske, E. J.; Dopfer, O., High-Resolution Spectroscopy of Cluster Ions. Chem. Rev. 2000,
100, 3963-3998.
24. Boo, D. W.; Lee, Y. T., Infrared spectroscopy of the molecular hydrogen solvated
carbonium ions, CH+5(H2)n (n=1–6). J. Chem. Phys. 1995, 103, 520-530.
76
25. Johnson, C. J.; Wolk, A. B.; Fournier, J. A.; Sullivan, E. N.; Weddle, G. H.; Johnson, M.
A., Communication: He-tagged Vibrational Spectra of the SarGlyH
+
and H
+
(H2O)2,3 Ions:
Quantifying Tag effects in Cryogenic Ion Vibrational Predissociation (CIVP) Spectroscopy. J.
Chem. Phys. 2014, 140, 221101.
26. Topfer, M.; Schmid, P. C.; Kohguchi, H.; Yamada, K. M. T.; Schlemmer, S.; Asvany, O.,
Infrared photodissociation of cold CH3
+
-He2 complexes. Mol. Phys. 2019, 117, 1481-1485.
27. Asvany, O.; Markus, C. R.; Salomon, T.; Schmid, P. C.; Banhatti, S.; Brünken, S.;
Lipparini, F.; Gauss, J.; Schlemmer, S., High-resolution rovibrational spectroscopy of c-C3H2
+
:
The ν7 C–H antisymmetric stretching band. J. Mol. Struct. 2020, 1214, 128023.
28. Tsuge, M.; Tseng, C.-Y.; Lee, Y.-P., Spectroscopy of prospective interstellar ions and
radicals isolated in para-hydrogen matrices. Phys. Chem. Chem. Phys. 2018, 20, 5344-5358.
29. Jacox, M. E., The spectroscopy of molecular reaction intermediates trapped in the solid
rare gases. Chem. Soc. Rev. 2002, 31, 108-115.
30. Jacox, M. E.; Thompson, W. E., The infrared spectra of C2H4
+
and C2H3 trapped in solid
neon. J. Chem. Phys. 2011, 134, 064321.
31. Chen, S.-C.; Liu, M.-C.; Huang, T.-P.; Chin, C.-H.; Wu, Y.-J., Photodissociation and
infrared spectra of ethylene cations in solid argon. Chem. Phys. Lett. 2015, 630, 96-100.
32. Jacox, M. E.; Thompson, W. E., The production and spectroscopy of molecular ions
isolated in solid neon. Res. Chem. Intermed 1989, 12, 33-56.
33. Tanyag, R. M. P.; Jones, C. F.; Bernando, C.; O'Connell, S. M. O.; Verma, D.; Vilesov, A.
F., Experiments with large superfluid helium droplets. In Cold chemistry: Molecular scattering
and reactivity near absolute zero, Dulieu, O.; Osterwalder, A., Eds. Royal Society of Chemistry:
Cambridge, 2017; pp 401-455.
34. Toennies, J. P.; Vilesov, A. F., Superfluid Helium Droplets: A Uniquely Cold Nanomatrix
for Molecules and Molecular Complexes. Angewandte Chemie-International Edition 2004, 43,
2622-2648.
35. Choi, M. Y.; Douberly, G. E.; Falconer, T. M.; Lewis, W. K.; Lindsay, C. M.; Merritt, J.
M.; Stiles, P. L.; Miller, R. E., Infrared Spectroscopy of Helium Nanodroplets: Novel Methods for
Physics and Chemistry. Int Rev Phys Chem 2006, 25, 15-75.
77
36. Verma, D.; Tanyag, R. M. P.; O'Connell, S. M. O.; Vilesov, A. F., Infrared Spectroscopy
in Superfluid Helium Droplets. Advances in Physics-X 2018, 4, 1553569.
37. Callegari, C.; Ernst, W. E., Helium Droplets as Nanocryostats for Molecular Spectroscopy
- from the Vacuum Ultravoilet to the Microwave regime. In Handbook of High-resolution
Spectroscopy, Quack, M.; Merkt, F., Eds. John Wiley & Sons, Ltd.: 2011; pp 1551-1594.
38. Smolarek, S.; Brauer, N. B.; Buma, W. J.; Drabbels, M., IR Spectroscopy of Molecular
Ions by Nonthermal Ion Ejection from Helium Nanodroplets. J. Am. Chem. Soc. 2010, 132, 14086-
14091.
39. Bierau, F.; Kupser, P.; Meijer, G.; von Helden, G., Catching Proteins in Liquid Helium
Droplets. Phys. Rev. Lett. 2010, 105, 133402.
40. González Flórez, A. I.; Ahn, D. S.; Gewinner, S.; Schollkopf, W.; von Helden, G., IR
Spectroscopy of Protonated Leu-enkephalin and its 18-crown-6 Complex Embedded in Helium
Droplets. Phys. Chem. Chem. Phys. 2015, 17, 21902-21911.
41. Lewerenz, M.; Schilling, B.; Toennies, J. P., Successive capture and coagulation of atoms
and molecules to small clusters in large liquid-helium clusters. J. Chem. Phys. 1995, 102, 8191-
8207.
42. Mauracher, A.; Echt, O.; Ellis, A. M.; Yang, S.; Bohme, D. K.; Postler, J.; Kaiser, A.;
Denifl, S.; Scheier, P., Cold physics and chemistry: Collisions, ionization and reactions inside
helium nanodroplets close to zero K. Physics Reports-Review Section of Physics Letters 2018, 751,
1-90.
43. Kuhn, M.; Renzler, M.; Postler, J.; Ralser, S.; Spieler, S.; Simpson, M.; Linnartz, H.;
Tielens, A. G. G. M.; Cami, J.; Mauracher, A.; Wang, Y.; Alcami, M.; Martin, F.; Beyer, M. K.;
Wester, R.; Lindinger, A.; Scheier, P., Atomically Resolved Phase Transition of Fullerene Cations
Solvated in Helium Droplets. Nat.Comm. 2016, 7, 13550.
44. Davies, J. A.; Besley, N. A.; Yang, S. F.; Ellis, A. M., Probing Elusive Cations: Infrared
Spectroscopy of Protonated Acetic Acid. J. Phys. Chem. Lett. 2019, 10, 2108-2112.
45. Davies, J. A.; Besley, N. A.; Yang, S.; Ellis, A. M., Infrared spectroscopy of a small ion
solvated by helium: OH stretching region of HeN−HOCO
+
. J. Chem. Phys. 2019, 151, 194307.
78
46. Verma, D.; Erukala, S.; Vilesov, A., Infrared Spectroscopy of Water and Zundel cations in
Helium Nanodroplets. J. Phys. Chem. A 2020, 124, 6207-6213.
47. Erukala, S.; Verma, D.; Vilesov, A., Rotation of CH3
+
Cations in Helium Droplets. J. Phys.
Chem. Lett 2021, 12, 5105-5109.
48. Verma, D.; Vilesov, A. F., Pulsed Helium Droplet Beams. Chem. Phys. Lett. 2018, 694,
129-134.
49. Yang, S. F.; Ellis, A. M., Helium droplets: A chemistry perspective. Chem. Soc. Rev. 2013,
42, 472-484.
50. Xing, X.; Bahng, M.-K.; Wang, P.; Lau, K.-C.; Baek, S. J.; Ng, C. Y., Rovibrationally
selected and resolved state-to-state photoionization of ethylene using the infrared-vacuum
ultraviolet pulsed field ionization-photoelectron method. J. Chem. Phys. 2006, 125, 133304.
51. Douberly, G. E.; Ricks, A. M.; Ticknor, B. W.; McKee, W. C.; Schleyer, P. v. R.; Duncan,
M. A., Infrared Photodissociation Spectroscopy of Protonated Acetylene and Its Clusters. J. Phys.
Chem. A 2008, 112, 1897-1906.
52. Wang, H. M.; Neese, C. F.; Morong, C. P.; Kleshcheva, M.; Oka, T., High-Resolution
Near-Infrared Spectroscopy of CH2
+
and Its Deuterated Isotopologues. J. Phys. Chem. A 2013,
117, 9908-9918.
53. Tiefenthaler, L.; Ameixa, J.; Martini, P.; Albertini, S.; Ballauf, L.; Zankl, M.; Goulart, M.;
Laimer, F.; Haeften, K. v.; Zappa, F.; Scheier, P., An intense source for cold cluster ions of a
specific composition. Rev. Sci. Intrum. 2020, 91, 033315.
54. Wallace, W. E., Mass Spectra. In NIST Chemistry WebBook, NIST Standard Reference
Database Number 69, Linstorm, P. J.; Mallard, W. G., Eds. National Institute of Standards and
Technology, Gaithersburg MD, 20899.
55. Anicich, V. G., An index of the literature for biomolecular gas phase cation-molecule
reaction kinetics. In JPL publication-03-19, JPL publication-03-19: U.S., 2003.
56. Albertini, S.; Gruber, E.; Zappa, F.; Krasnokutski, S.; Laimer, F.; Scheier, P., Chemistry
and physics of dopants embedded in helium droplets. Mass Spectrom. Rev. 2021.
79
57. Braun, A.; Drabbels, M., Photodissociation of alkyl iodides in helium nanodroplets. I.
Kinetic energy transfer. J. Chem. Phys. 2007, 127, 114303.
58. Dopfer, O.; Luckhaus, D., Rovibrational calculations for CH3
+
–Rg (Rg=He,Ne): The
prototype disk-and-ball dimer. J. Chem. Phys. 2002, 116, 1012-1021.
59. Hartmann, M.; Portner, N.; Sartakov, B.; Toennies, J. P.; Vilesov, A. F., High resolution
infrared spectroscopy of single SF6 molecules in helium droplets. I. Size effects in
4
He droplets. J.
Chem. Phys. 1999, 110, 5109-5123.
60. Wallace, W. E., Infrared Spectra. In NIST Chemistry WebBook, NIST Standard Reference
Database Number 69, Linstorm, P. J.; Mallard, W. G., Eds. National Institute of Standards and
Technology, Gaithersburg MD, 20899.
61. Dopfer, O.; Olkhov, R. V.; Mladenović, M.; Botschwina, P., Intermolecular interaction in
an open-shell π-bound cationic complex: IR spectrum and coupled cluster calculations for C2H2
+
-
Ar. J. Chem. Phys. 2004, 121, 1744-1753.
62. Yeh, L. I.; Okumura, M.; Myers, J. D.; Price, J. M.; Lee, Y. T., Vibrational Spectroscopy
of the Hydrated Hydronium Cluster Ions H3O
+
.(H2O)n (n = 1, 2, 3). J. Chem. Phys. 1989, 91, 7319-
7330.
63. Ayotte, P.; Bailey, C. G.; Kim, J.; Johnson, M. A., Vibrational predissociation spectroscopy
of the (H2O)6
−
⋅Arn, n⩾6, clusters. J. Chem. Phys. 1998, 108, 444-449.
64. Hartmann, M.; Miller, R. E.; Toennies, J. P.; Vilesov, A., Rotationally resolved
spectroscopy of SF6 in liquid-helium clusters - A molecular probe of cluster temperature. Phys.
Rev. Lett. 1995, 75, 1566-1569.
65. Slipchenko, M. N.; Vilesov, A. F., Spectra of NH3 in He droplets in the 3 μm range. Chem.
Phys. Lett. 2005, 412, 176-183.
66. Hoshina, H.; Skvortsov, D.; Sartakov, B. G.; Vilesov, A. F., Rotation of methane and silane
molecules in He droplets. J. Chem. Phys. 2010, 132, 074302.
67. Gomez, L. F.; Sliter, R.; Skvortsov, D.; Hoshina, H.; Douberly, G. E.; Vilesov, A. F.,
Infrared Spectra in the 3 μm Region of Ethane and Ethane Clusters in Helium Droplets. J. Phys.
Chem. A 2013, 117, 13648-13653.
80
68. Lindsay, C. M.; Miller, R. E., Rotational and vibrational dynamics of ethylene in helium
nanodroplets. J. Chem. Phys. 2005, 122, 104306.
69. Xing, X.; Reed, B.; Bahng, M.-K.; Ng, C. Y., Infrared−Vacuum Ultraviolet Pulsed Field
Ionization-Photoelectron Study of C2H4
+
Using a High-Resolution Infrared Laser. J. Phys. Chem.
A 2008, 112, 2572-2578.
70. Hoshina, H.; Slipchenko, M.; Prozument, K.; Verma, D.; Schmidt, M. W.; Ivanic, J.;
Vilesov, A. F., Infrared spectroscopy and structure of (NO)n clusters. J. Phys. Chem. A 2016, 120,
527-534.
71. Willitsch, S.; Merkt, F., Characterization of the X ̃
2
A1 (0,0,0) ground vibronic state of CH2
+
by pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy. J. Chem. Phys. 2003,
118, 2235-2241.
72. Kuyanov, K. E.; Slipchenko, M. N.; Vilesov, A. F., Spectra of the ν1 and ν3 Bands of Water
Molecules in Helium Droplets. Chem. Phys. Lett. 2006, 427, 5-9.
81
Chapter 5. Infrared Spectroscopy of Ions and Ionic Clusters upon
Ionization of Ethane in Helium Droplets
This chapter is based on publication by S. ERUKALA, A.F. FEINBERG, C.J. MOON, M.Y. CHOI
AND A.F. VILESOV, “Infrared Spectroscopy of Ions and Ionic Clusters upon Ionization of Ethane
in Helium droplets”, J..Chem. Phys., 125, 204306 (2022)
Helium droplets are unique hosts for isolating diverse molecular ions for infrared
spectroscopic experiments. Recently, it was found that electron impact ionization of ethylene
clusters embedded in helium droplets produces diverse carbocations containing three and four
carbon atoms, indicating effective ion-molecule reactions. In this work, similar experiments are
reported but with saturated hydrocarbon precursor, such as ethane. In distinction to ethylene
precursors, no characteristic bands of larger covalently bound carbocations were found, indicating
inefficient ion-molecule reactions. Instead, the ionization in helium droplets leads to formation of
weaker bound dimers such as (C2H6)(C2H4)
+
, (C2H6)(C2H5)
+
, and (C2H6)(C2H6)
+
, as well as larger
clusters containing several ethane molecules attached to C2H4
+
, C2H5
+
, and C2H6
+
ionic cores. The
spectra of larger clusters resemble those for neutral, neat ethane clusters. This work shows the
utility of the helium droplets to study small ionic clusters at ultra-low temperatures.
5.1 Introduction
Carbocations are ubiquitous reactive intermediates in the condensed and gas phases,
underscoring the importance of knowledge on their structure and reactivity.
1-3
Considerable efforts
have been invested into developing techniques to interrogate carbocations by infrared
spectroscopy, such as in discharge
4
or solid matrices.
5-7
Spectroscopy in molecular beams
8-11
and
more recently, in cryogenic traps
12-14
often involves tagging ions with rare gas atoms. Spectroscopy
of ions in helium droplets provides an attractive technique that was developed by several groups.
82
15-19
Due to weak interactions between the ions and host liquid helium, and thermalization to the
low temperature (0.4 K) of the droplet, narrow vibrational bands of order few wavenumbers
have
been observed for different cations. We have shown that ions solvated in droplets of few thousand
helium atoms can be obtained upon electron impact ionization of droplets doped with neutral
precursors, such as methane and ethylene.
17,20, 21
We have also found that ionization of ethylene
molecules and clusters in helium droplets, in addition to formation of small fragment ions (such
as CH2
+
, CH3
+
, C2H2
+
, and C2H3
+
) yields larger cations (such as C3Hn
+
and C4Hm
+
) which stem
from ion-molecule reactions between ethylene molecules and ions. No additional bands stemming
from complexes of ions with ethylene molecules were identified, indicating high efficiency of the
ion-molecule reactions. Thus, it is interesting to see if similar larger carbocations are also formed
from saturated molecules such as ethane, which is in the focus of this work.
We find that ionization of ethane molecules leads to the formation of CH2
+
, C2H4
+
, C2H3
+
,
C2H4
+
, and C2H5
+
fragment ions. Ionization of ethane dimers and clusters in helium droplets does
not produce any noticeable amounts of larger carbocations such as C3Hn
+
and C4Hm
+
which were
previously detected with ethylene precursor.
21
Instead, the ionization yields clusters of several
ethane molecules with C2H4
+
, C2H5
+
, and C2H6
+
ions. This shows that for saturated molecules like
ethane, ion-molecular reactions are much less effective. Additionally, it highlights the possibility
of studying larger ionic clusters at ultracold temperature, such as water clusters, which have
previously been studied extensively in molecular beams at higher temperatures.
5.2 Experimental set up
The experimental apparatus used to measure infrared spectra of ions in helium droplets is
described in Ref.
17
. Helium droplet pulses of width ~ 200 s are produced upon the expansion of
83
helium gas into vacuum through a 1 mm diameter pulsed nozzle (General Valve series 99). The
droplets employed in this work have an average size of ~7000 atoms corresponding to a stagnation
pressure of P0 = 20 bar and a nozzle temperature of T0 = 23 K. After exiting the nozzle, the droplets
enter the 44 cm long pickup chamber through a 2 mm skimmer where they capture ethane
molecules. Ethane backfills the entire vacuum chamber. Its pressure is regulated by a leak valve
and measured an ionization pressure gauge. Throughout the paper, the quoted pickup pressures
correspond to the nominal reading of the ion gauge. The absolute pressure could be obtained by
dividing the reading by the sensitivity coefficient for ethane of 2.6. The doped droplets continue
traveling further downstream until they enter the detection chamber, which hosts an Extrel MAX
500 quadrupole mass spectrometer (QMS). The QMS has an additional axial external ionizer
placed ~ 20 cm upstream from the ion range as explained in Ref
17
. The droplets are ionized with
the axial external ionizer set to 100 eV, 10 mA. For the rest of the manuscript, the axial ionizer is
referred to as the external ionizer, whereas the main ionizer is referred to as the probe ionizer.
The doped, ionic droplets are irradiated by a focused infrared laser beam into the ion region
of the probe ionizer of the QMS. The laser enters the vacuum system anti-collinear with the droplet
beam through the calcium fluoride window on the axis of the QMS chamber
21
. Some small
fraction of the laser flux likely reaches the face of the pulse nozzle valve. However, we do not
observe any effect of the nozzle heating by the laser as for example was detected in our previous
work with the continuous nozzle and collimated pulsed laser beam.
22
In comparison only a weak
portion of the divergent laser beam can reach the nozzle in this work. Besides the pulse of the
droplets is produced about 2.7 ms ahead of the laser pulse.Free ions are released upon absorption
of several infrared quanta. These free ions are extracted, mass selected, and detected by the QMS.
17
Infrared spectra are recorded by monitoring the gated (~ 10 μs) QMS signal of the bare ions
84
generated. The spectra are obtained using a pulsed optical parametric oscillator-amplifier (Laser
Vision, spectral resolution: ~ 0.1 cm
-1
, pulse energy ~ 5 - 8 mJ, repetition rate 20 Hz) pumped by
a Nd:YAG pulsed laser of ~7 ns pulse width. The absolute frequency of the laser is calibrated
using the photo-acoustic spectrum of methane and ammonia molecules.
5.3 Results
Figure 5.1. The mass spectrum obtained in probe ionizer of QMS upon doping He droplets with ethane at
4×10
-6
mbar ethane. The intensity of the most intense peaks around 6000 is incorrect due to saturation of
the detection system.
Figure 5.1 shows the mass spectrum of the helium droplets doped with ethane at 4×10
-6
mbar upon ionization in the probe ionizer of the QMS.
21, 23
The intensity was recorded using boxcar
integrator with a gate of width ~150 s delayed by ~3.07 ms with respect to the trigger of the
pulsed valve. It contains a sequence of HeN
+
peaks decreasing in intensity with N. The additional
peaks due to OH
+
, H2O
+
, N2
+
, and CO2
+
stem from the ionization of the rest gas present in the
QMS chamber. Intense peaks of C2H2
+
, C2H3
+
, C2H4
+
, C2H5
+
, arising from ionization and
fragmentation of C2H6 are observed. The fragments we observe, which originate from within the
85
helium droplets, are ejected from the droplets upon ionization. In addition to smaller ions, peaks
corresponding to ions containing three and four carbon atoms are observed. The intensity of some
mass peaks versus pickup pressure have been studied similar to Ref.
21
as exemplified in the SM.
The intensity of the small ions C2H3
+
, C2H4
+
, C2H5
+
, and C2H6
+
reach a maximum at 3.1×10
-6
,
4.5×10
-6
, 8×10
-6
, and 1.2×10
-5
mbar, respectively. The smallest of the pressures for C2H3
+
gives
the best upper boundary estimate for the pressure required for the pickup of single ethane
molecules and is close to the value found in our previous study for the ionization of ethylene
molecules. Higher maximum pressures for larger ions indicate that they mostly stem from small
clusters of ethane, such as dimers and trimers. In comparison, the intensity of the larger ions C4Hm
+
(m = 9-12) reaches a maximum at a much higher pressure of about 2×10
-5
mbar (red trace),
indicating they stem from larger clusters. The larger ions containing three or four carbon atoms
may be either covalently bound resulting from ion-molecule reactions, or clusters of smaller ions
bound with C2H6 by inductive forces.
Figure 5.2. Total ion yield upon laser irradiation of helium droplets doped at 4×10
-6
(black trace) and
2×10
-5
mbar (red trace) ethane pickup pressure.
86
Figure 5.2 shows the spectrum of the total ion yield upon laser irradiation of doped droplets
which were irradiated by the electron beam in the external ionizer. During these measurements,
the DC of the QMS poles was off and the RF was set to transmit all ions with m/e > 4 au. The
boxcar gate was set to 150 μs to accept both light and heavy ions with different time of flight
through the QMS. The black trace in Fig. 5.2 is collected at 4×10
-6
mbar pickup pressure. The most
intense peaks at 3006.5, 3105.2, 3145.5 cm
-1
are assigned to v11 and v9 band of C2H4
+
and v6 band
of C2H3
+
, respectively. The assignment is in agreement with the frequencies reported for ions from
ethylene precursor in helium droplets as in Ref
21
. The mass resolved measurements, which were
described previously,
21
show that weaker peaks at 3122.9 , 3150.2, and 3160 cm
-1
stem from mass
M=29, which could be assigned to C2H5
+
.
21
Some other weaker peaks are assigned in Fig. 5.2 by
the masses observed upon laser irradiation. Unlike with ethylene precursor, peaks from
carbocations containing three and four carbon atoms were not detected, implying inefficient ion
molecule reactions involving saturated ethane molecules. Instead, ethane forms clusters at higher
pick-up pressures, which are in the focus of the present work.
The red trace in Fig.5.2 shows the total ion yield spectrum collected at 2×10
-5
mbar pressure
of ethane, corresponding to pickup of about five ethane molecules. The spectrum has prominent
peaks at 2811.3, 2877.1, 2943.2, and 2987.2 cm
-1
. For the assignment of these peaks, the laser
frequency was set to the maximum of a spectral peak and the mass spectrum was recorded with
the boxcar gate of 150 μs. The mass spectrum recorded with the laser set at 2987.2 cm
-1
is shown
with the red trace in Fig. 5.3. Groups of peaks around m/z = 60, 90, 120, 150, and 180 show that
the absorption at 2987.2 cm
-1
is due to ionic clusters containing few ethane molecules. The inset
shows the zoomed in plot around M=60. The peaks are assigned to dimers of C 2H4
+
, C2H5
+
, and
87
C2H6
+
with ethane molecules. Similarly, higher mass peaks are due to the clusters containing the
same ions with three, four and five ethane molecules. The red trace in Fig. 5.2 obtained at high
ethane pickup pressure shows no indication of the spectral peaks due to C2H4
+
or C2H5
+
, which
appear prominently at low pickup pressure. This likely indicates that the spectral lines due to C2H4
+
or C2H5
+
get broadened in ethane clusters. Thus, the broad prominent peaks in the red trace of Fig.
2 should be assigned to infrared absorption of ethane molecules in the ionic clusters. The peak at
2987.2 cm
-1
is assigned to the v7 band of C2H6 in ionic clusters based on its proximity to 2985.4
cm
-1
in free ethane and 2975 cm
-1
v7 band of C2H6 in neutral ethane clusters in helium droplets.
23
Similarly, peaks at 2877.1 and 2943.2 cm
-1
are assigned to the v5 and v8+11 bands of C2H6.
The spectral peak at 2811.3 cm
-1
in Fig. 5.2 could not be assigned to ethane clusters and
the corresponding signal has a different mass composition. The black trace in Fig. 5.3 shows the
mass spectrum measured upon excitation at 2811.3 cm
-1
. Instead of groups of peaks, it shows only
two prominent peaks at m/z = 30 and 60 which could be assigned to C2H6
+
and (C2H6)(C2H6)
+
.
Figure 5.3. Mass spectra recorded upon laser excitation at a) 2987.19 cm
-1
and b) 2811.3 cm
-1
respectively.
Ethane pickup pressure 2×10
-5
mbar.
88
Figure 5.4. Infrared spectra recorded upon detection of mass 30 (red trace) and 60 (Black trace) collected
at Ethane pickup pressure 2×10
-5
mbar.
Figure 5.4 shows the infrared spectra collected at M=30 and M=60. The black trace in Fig.
5.4 recorded at M=60 shows peaks at 2986, 2942, 2877, and 2811.3 cm
-1
having breadth of 5, 11,
3 and 11 cm
-1
respectively. The bands due to ionic clusters are broader than those due to molecular
ions such as CH3
+
,
20
C2H5
+
,
21
and H2O5
+ 17
in helium droplets which have width of 1-2 cm
-1
. The
red trace in Fig. 5.4 shows the spectrum obtained at M=30 with only one peak at 2811.3 cm
-1
which
appearance is like that recorded at M=60.
89
Figure 5.5. Pickup pressure dependences for laser induced signal as measured at indicated masses. a) and
b) were obtained with the laser tuned to the maximum of the v 7 band of ethane clusters at 2987 cm
-1
and c)
at 2811.3 cm
-1
. Curves are fits of the data points to Poisson probability of capturing k molecules per droplet.
Panels a) and b) of Fig. 5.5 show the dependence of the intensity of the v7 band versus
ethane pickup pressure obtained with the QMS tuned to M=58, 88, 59, and 89, respectively. The
masses correspond to the clusters containing C2H4
+
(a) and C2H5
+
(b) with one and two ethane
molecules. The curves are fits of the data points by Poisson probability:
𝐼 𝑘 (𝑃 ) = 𝐶 ∙
(𝑃 /𝐴 )
𝑘 𝑘 !
𝑒 −𝑃 /𝐴 (1)
for capture of k- ethane molecules per droplet, with A and C fitting parameters. In panels a) and
b), for the clusters with C2H4
+
and C2H5
+
, the intensity rises non–linearly at small pressures and
reaches a maximum at 1.5×10
-5
mbar and 2.5×10
-5
mbar for clusters with one (red trace) and two
(black trace) ethane molecules, respectively. The best fit is achieved for k=6, implying that the
clustering stems from ionization of larger moieties containing on average six ethane molecules.
Therefore, the observed smaller clusters result from extensive fragmentation either during
ionization, laser excitation or both. Panel c) in Fig. 5.5 shows the Poisson dependence of the signal
with laser set at 2811.3 cm
-1
for M=30 and M=60. The best fit is achieved for an average capture
90
of three ethane molecules per droplet with maximum at ~8×10
-6
mbar. Based on the requirement
for the pickup of single molecules of about 3×10
-6
mbar, the signal stems from small clusters,
around trimers.
Figure 5.6. Infrared spectra of the v 7 band of C 2H 6 in ionic clusters as recorded at of a) M=90, 89, 88
and b) M=60, 59 and 58. The frequencies of the v 7 band in neutral ethane clusters in helium droplets and
in free molecules are shown by green and black sticks, respectively.
Figure 5.6 shows the spectra of v7 band of ethane clusters with C2H6
+
ions (M=60, 90-black
trace), C2H5
+
(M=59, 89 – red trace) and C2H4
+
(M=58, 88 – blue trace). The total ion yield spectra
in the range of 2700 – 3100 cm
-1
are shown in Fig. S2 of SM. It is seen that the peak from
(C2H5
+
)(C2H6)n clusters (red trace) is the most intense, whereas the peak from (C2H6
+
)(C2H6)n
clusters (black trace) is the narrowest. The maxima of the peaks are very close. The peak due to
ethane clusters containing C2H6
+
ions (black trace) is redshifted by ~2 cm
-1
with respect to C2H5
+
(red trace) and C2H4
+
(blue trace). The frequency of the v7 band in ionic clusters is very close to
that in free molecules (black stick).
24
The v7 band in large neutral ethane clusters containing about
6000 molecules in helium droplets blue shifted by ~ 10 – 12 cm
-1
(green stick).
23
91
5.4 Discussion
The mass spectrum in Fig. 5.1 shows the formation of diverse free ions upon electron
impact ionization of the helium droplets doped with ethane molecules. In addition, a considerable
fraction of ions are retained in the droplets, however the branching ratio between the ejection and
retainment remains unknown and is likely unique for each ion. The infrared spectrum upon
ionization of helium droplets containing, on average, single ethane molecules (black trace in Fig.
5.2) indicates that C2H2
+
, C2H3
+
, C2H4
+
,
and C2H5
+
and to smaller extent CH2
+
are the dominant
fragment ions that are retained within the droplets upon the ionization. However, Fig. 5.2 shows
no bands from larger ions (C3Hn
+
, C4Hm
+
) which may result from secondary ion–molecule
reactions of the primary ions with ethane molecules. Such larger ions were observed in our
previous study with ethylene precursor. This difference likely reflects a smaller reactivity of the
ions with saturated ethane molecules. It is interesting that some larger free ions such as C 3H5
+
,
C3H6
+
, C3H7
+
and C3H8
+
are seen in Fig. 5.1, some of them may be bound covalently. It probably
shows that ion molecule reactions with ethane molecules do proceed to some extent, but the
abundance of the products staying within the droplets is too small to be detected in the infrared
spectra.
Fig. 5.3 shows that at higher doping pressures of ethane, the ionization of the ethane
clusters leads to the formation of non-covalently bound clusters of C2H4
+
, C2H5
+
, and C2H6
+
ions
with several ethane molecules. The IR transition frequencies of clusters of ethane with C2H4
+
,
C2H5
+
, and C2H6
+
ions are similar to that of neutral ethane clusters in helium droplets.
23
The widths
of the bands (~10 cm
-1
) are also comparable to that observed in neutral clusters. Figures 5.3 and
5.6 show that the intensity of the v7 band is higher for clusters containing C2H5
+
, which indicates
their predominant formation upon ionization of the ethane clusters. On the other hand, Fig. 5.3
92
shows that the signal from clusters containing C2H3
+
at M=57 and 87 is rather weak, possibly
indicating inefficient formation of clusters with C2H3
+
upon ionization.
The mass spectrum in Fig. 5.3 (red trace) obtained with laser set to v7 shows peaks due to
clusters of C2H4
+
, C2H5
+
, and C2H6
+
ions with ethane at masses around 60, 90, 120, 150 and 180
au. The signal from these clusters comes from the pick-up of at least 6 ethane molecules as seen
from the Poisson dependences in Figs. 5.5a and 5.5b. However, the mass spectrum of the ionic
clusters in Fig. 5.3 shows that the most intense ions correspond to clusters of C2H4
+
, C2H5
+,
and
C2H6
+
ionic cores with one and two ethane molecules attached, around masses of 60 and 90 au.
This likely indicates that the small clusters observed result from the fragmentation of larger ionic
clusters. The fragmentation is facilitated by the large width of the infrared bands of about 10 cm
-
1
, and the fact that there is only a small difference in the frequency of the bands for clusters of
different size. Thus, the v7 band in clusters of different size remain in resonance with the laser,
which assures an effective pumping of laser energy. It is noticeable that no signal from bare C2H4
+
,
C2H5
+
and C2H6
+
ions is observed in Fig. 5.3. This may indicate that the laser photon energy
(2987.2 cm
-1
) becomes lower than the binding energy of the dimers of C2H4
+
, C2H5
+
, and C2H6
+
ions with ethane. This is in agreement with the reported enthalpy of formation of ~5350 cm
-1
for
(C2H4) (C2H6)
+
clusters.
25
The fall of intensity of masses beyond mass 90 in Fig. 5.3 indicates a
decrease in binding energy with an increase in cluster size. Although we did not find the results
for ethane clusters, the formation enthalpies for C2H4
+
with a different number of ethylene
molecules are available. The data shows that the formation enthalpy sharply drops from ~5500 cm
-
1
for C2H4
+
-C2H4 to ~1500 cm
-1
for (C2H4
+
-C2H4)- C2H4.
26, 27
IR frequencies of ions resulting from C2H6 fragmentation seen in Fig. 5.2 (black trace) such
as CH2
+
, C2H2
+
, C2H3
+
, C2H4
+
, and CH2
+
are observed at the same frequencies as obtained
93
previously with ethylene precursor.
21
Besides those previously identified bands, the spectrum in
Fig. 5.2 shows some weaker features. The spectra recorded at different masses are shown in Fig.
S2 of SM. The bands marked by M’s have been studied at some detail. Excitation of some of those
bands yields signal at different masses, such as M=29, 57 and 59 for 2848 cm
-1
, M=28, 57 for 3016
cm
-1
and M=29, 57 for 3165 cm
-1
. Only ions with M=29 were observed upon excitation of the
3122, 3150, and 3160 cm
-1
bands. The ethane pickup pressure dependence has been studied upon
excitation at 2811.25 cm
-1
with detection at M=30 and 60 and 3150 at M=29 yielding maxima at
~8×10
-6
mbar and ~2.5×10
-6
mbar, respectively. The total ion spectra measured at different pickup
pressures indicates that the bands at 3150, 3160, and 3165 cm
-1
have similar pressure dependence,
whereas the bands at 2848 and 3122 cm
-1
reaches maxima at intermediate pressure. The bands that
show signal at several masses must come from small ionic clusters which fragment upon laser
excitation, with the smallest mass corresponding to the ionic core such as C2H4
+
, C2H5
+
and C2H6
+
for M=28, 29, and 30, respectively. The observation of the higher mass peaks agrees with adducts
such as C2H4 and C2H6. Expansion of spectroscopy in helium droplets to ionic dimers opens
interesting perspectives. However, more work is required to measure the full spectra of the ionic
dimers, which is beyond the scope of the present paper. Here we limit the discussion to a few
specific bands.
Fig. 5.3 and 5.4 show rather unique behavior of the signal at mass channel M=60, which
besides the bands of ethane clusters at 2877, 2943 and 2987 cm
-1
, shows a pronounced band at
2811.3 cm
-1
having width of about 10 cm
-1
. In addition, Figs. 5.3 and S3 in the SM show that
excitation at 2811.3 cm
-1
also gives rise to a signal at M=30, and some weak signal at M=90, but
no larger ions such as M=120 or 150. No band around 2811.3 cm
-1
was observed in neutral ethane
clusters.
24
Figure 5.5 (c) shows that its maximum is achieved at lower pickup pressure of ethane
94
than the bands of clusters in Fig. 5.5 (a,b). The Poisson fit in Fig. 5.5 (c) shows that the ions
responsible for the 2811.3 cm
-1
peak originate from an average pickup of three molecules per
droplet for both mass channel 30 and 60. Thus the 2811.3 cm
-1
band likely stems from ionized
dimers or trimers of ethane.
It is interesting that we were not able to identify any additional bands at M=30 which could
be assigned to single C2H6
+
ions in helium droplets. The structure and dynamics of the C2H6
+
has
been extensively discussed in the literature.
28-33
Its ground state corresponds to
2
Ag (C 2h) and
results from Jahn-Teller distortion of the
2
Eg (D3h) electronic state. Information on the C2H6
+
came
from photoelectron spectroscopy, electron paramagnetic resonance spectroscopy and ab initio
calculations, however it was never observed in infrared spectroscopy. Ethane ions were
extensively discussed in the literature as an example of a fluxional molecule,
29
and their spectra
may have a considerable breadth due to manifold or transitions involving different tunneling states.
The detection of the broad spectra with correspondingly small peak absorption cross section is
inefficient in this work involving multiple photon excitation. The band at 2811.3 cm
-1
likely
corresponds to C2H6-C2H6
+
dimers. The band has width of about 10 cm
-1
, but appears prominently
in the spectrum, and should likely have a substantial infrared intensity of several hundreds of
km/mol. The origin of this unusually strong band warrants more studies in the future.
Infrared spectra measured at M=29 which is shown in SM, Fig. S3, revealed a sharp band
at 3122.2 cm
-1
and two close features at 3150 and 3160 cm
-1
. The Poisson dependencies indicated
that the first band is associated with the ionization of two ethane molecules, whereas the two bands
come upon ionization of single ethane molecules. The same bands were also observed at M=29
during our previous study with ethylene precursor.
21
Different pressure dependencies of the bands
indicates that they stem from different species. However, their identification is not immediately
95
clear. Mass 29 corresponds to the ethyl carbocation C2H5
+
, which is a fundamental ion whose
structure has been discussed to be either a H3C-CH2
+
classical or a bridged non-classical C2H5
+
.
Infrared spectra of C2H5
+
tagged with argon atoms have been studied by infrared
photodissociation.
34, 35
The experiments reported asymmetric and symmetric stretches of CH2 units
of C2H4 entity in bridged C2H5
+
at 3117 and 3037 cm
-1
respectively. The peaks are broadly within
the range of sp
2
C-H stretch. No bands in the 2900-3000 cm
-1
region corresponding to sp
3
C-H
stretch were observed.
34
In addition, a bridged proton stretch has been reported at 2158 cm
-1
,
36
which is beyond the range accessible in the present work. A recent helium droplet study reported
asymmetric and symmetric stretches of the CH2 units of the non-classical C2H5
+
at 3122 and 3038
cm
-1
respectively, as well as two other bands at lower frequency assigned to combination bands.
37
In this work, IR peaks for
M=29 are observed at 3122.3, 3150.2, and 3159.8 cm
-1
, respectively, as
shown in Fig. S3 of SM. The peak at 3122.3 cm
-1
can be assigned to the asymmetric stretch of CH2
units of the bridged C2H5
+
. However, no low frequency peak due to symmetric stretching was
observed in the vicinity of ~3038 cm
-1
, which is puzzling. Similarly, no peak was observed in the
sp
3
C-H stretch range of 2900-3000 cm
-1
which may correspond to the classical isomer.
36
The
identity of the peaks at 3150.2 and 3159.8 remains unclear. One possibility is that they may stem
from N2H
+
impurities, which N-H stretch frequency was found to be at 3158 and 3164 cm
-1
when
obtained upon tagging with one and two helium atoms, respectively.
38, 39
We performed separate
experiments in which nitrogen gas was added to ethane or hydrogen gas in the pickup chamber.
However, those experiments did not yield any bands at 3158 or 3164 cm
-1
.Further work is required
to assign the peaks observed for C2
96
5.5 Conclusions
This work continues our study on formation and infrared spectroscopy of carbocations in
helium droplets. Ionization of ethane in helium droplets produces a number of embedded fragment
ions, such as CH2
+
, C2H2
+
, C2H3
+
, and C2H4
+
which the same frequencies of the infrared bands as
previously observed upon ionization of ethylene in helium droplets. However, in distinction to
ethylene precursors, no characteristic bands of larger covalently bound carbocations with three and
four carbon atoms were found, indicating inefficient ion-molecule reactions involving saturated
ethane molecules. Ionization of ethane clusters in helium droplets leads to formation of weaker
bound dimers such as (C2H6)(C2H4)
+
, (C2H6)(C2H5)
+
, and(C2H6)(C2H6)
+
as well as larger clusters
containing several ethane molecules attached to C2H4
+
, C2H5
+
, and C2H6
+
ionic cores. The
vibrational frequencies of the larger ionic clusters are close to those observed in neutral ethane
clusters. Several weaker bands are temporarily assigned to non-covalent dimers of the primary
ions with ethane molecules. This work shows the utility of the helium droplets to study small ionic
clusters at ultra-low temperatures
5.6 References
1. Shuman, N. S.; Hunton, D. E.; Viggiano, A. A., Ambient and Modified Atmospheric Ion
Chemistry: From Top to Bottom. Chemical Reviews 2015, 115, 4542-4570.
2. Olah, G. A.; Prakash, G. K. S.; Molnar, A.; Sommer, J., Superacid Chemistry. John Wiley
& Sons: Hoboken, New Jersey, 2009.
3. Tielens, A. G. G. M., The Physics and Chemistry of the Interstellar Medium. Cambridge
University Press: Cambridge, 2005.
4. Jensen, P.; Brumm, M.; Kraemer, W. P.; Bunker, P. R., An Ab-Initio Calculation of the
Rovibronic Energies of the CH2
+
Molecule. J. Mol. Spectrosc. 1995, 172, 194-204.
97
5. Tsuge, M.; Tseng, C.-Y.; Lee, Y.-P., Spectroscopy of prospective interstellar ions and
radicals isolated in para-hydrogen matrices. Phys. Chem. Chem. Phys. 2018, 20, 5344-5358.
6. Jacox, M. E., The spectroscopy of molecular reaction intermediates trapped in the solid
rare gases. Chem. Soc. Rev. 2002, 31, 108-115.
7. Chen, S.-C.; Liu, M.-C.; Huang, T.-P.; Chin, C.-H.; Wu, Y.-J., Photodissociation and
infrared spectra of ethylene cations in solid argon. Chem. Phys. Lett. 2015, 630, 96-100.
8. Roithova, J.; Gray, A.; Andris, E.; Jasik, J.; Gerlich, D., Helium Tagging Infrared
Photodissociation Spectroscopy of Reactive Ions. Acc. Chem. Res. 2016, 49, 223-230.
9. Duncan, M. A., Infrared Laser Spectroscopy of Mass-Selected Carbocations. J. Phys.
Chem. A 2012, 116, 11477-11491.
10. Bieske, E. J.; Dopfer, O., High-Resolution Spectroscopy of Cluster Ions. Chem. Rev. 2000,
100, 3963-3998.
11. Boo, D. W.; Lee, Y. T., Infrared spectroscopy of the molecular hydrogen solvated
carbonium ions, CH5
+
(H2)n (n=1–6). The Journal of Chemical Physics 1995, 103, 520-530.
12. Johnson, C. J.; Wolk, A. B.; Fournier, J. A.; Sullivan, E. N.; Weddle, G. H.; Johnson, M.
A., Communication: He-tagged Vibrational Spectra of the SarGlyH
+
and H
+
(H2O)2,3 Ions:
Quantifying Tag effects in Cryogenic Ion Vibrational Predissociation (CIVP) Spectroscopy. J.
Chem. Phys. 2014, 140, 221101.
13. Asvany, O.; Brunken, S.; Kluge, L.; Schlemmer, S., COLTRAP: A 22-pole ion trapping
machine for spectroscopy at 4 K. Applied Physics B-Lasers and Optics 2014, 114, 203-211.
14. Asvany, O.; Kumar, P.; Redlich, B.; Hegemann, I.; Schlemmer, S.; Marx, D.,
Understanding the infrared spectrum of bare CH5
+
. Science 2005, 309, 1219-1222.
15. Smolarek, S.; Brauer, N. B.; Buma, W. J.; Drabbels, M., IR Spectroscopy of Molecular
Ions by Nonthermal Ion Ejection from Helium Nanodroplets. J. Am. Chem. Soc. 2010, 132, 14086-
14091.
98
16. González Flórez, A. I.; Ahn, D. S.; Gewinner, S.; Schollkopf, W.; von Helden, G., IR
Spectroscopy of Protonated Leu-enkephalin and its 18-crown-6 Complex Embedded in Helium
Droplets. Phys. Chem. Chem. Phys. 2015, 17, 21902-21911.
17. Verma, D.; Erukala, S.; Vilesov, A., Infrared Spectroscopy of Water and Zundel cations in
Helium Nanodroplets. J. Phys. Chem. A 2020, 124, 6207-6213.
18. Davies, J. A.; Besley, N. A.; Yang, S.; Ellis, A. M., Probing Elusive Cations: Infrared
Spectroscopy of Protonated Acetic Acid. The journal of physical chemistry letters 2019, 10, 2108-
2112.
19. Kuhn, M.; Renzler, M.; Postler, J.; Ralser, S.; Spieler, S.; Simpson, M.; Linnartz, H.;
Tielens, A. G.; Cami, J.; Mauracher, A.; Wang, Y.; Alcami, M.; Martin, F.; Beyer, M. K.; Wester,
R.; Lindinger, A.; Scheier, P., Atomically resolved phase transition of fullerene cations solvated
in helium droplets. Nature communications 2016, 7, 13550.
20. Erukala, S.; Verma, D.; Vilesov, A., Rotation of CH3
+
Cations in Helium Droplets. J. Phys.
Chem. Lett 2021, 12, 5105-5109.
21. Erukala, S.; Feinberg, A.; Singh, A.; Vilesov, A. F., Infrared spectroscopy of carbocations
upon electron ionization of ethylene in helium nanodroplets. J. Chem. Phys. 2021, 155, 084306.
22. Gomez, L. F.; Loginov, E.; Sliter, R.; Vilesov, A. F., Sizes of large He droplets. J. Chem.
Phys. 2011, 135, 154201.
23. Gomez, L. F.; Sliter, R.; Skvortsov, D.; Hoshina, H.; Douberly, G. E.; Vilesov, A. F.,
Infrared Spectra in the 3 μm Region of Ethane and Ethane Clusters in Helium Droplets. J. Phys.
Chem. A 2013, 117, 13648-13653.
24. Hepp, M.; Herman, M., Weak Combination Bands in the 3-μm Region of Ethane. J. Mol.
Spectrosc. 1999, 197, 56-63.
25. Hiraoka, K.; Kebarle, P., Ion molecule reactions in ethane. Thermochemistry and structures
of the intermediate complexes: C4H11+ and C4H10+ formed in the reactions of C2H5+ and
C2H4+ with C2H6. Can. J. Chem. 1980, 58, 2262-2270.
26. Ceyer, S. T.; Tiedemann, P. W.; Ng, C. Y.; Mahan, B. H.; Lee, Y. T., Photoionization of
ethylene clusters. J. Chem. Phys. 1979, 70, 2138-2144.
99
27. Ono, Y.; Linn, S. H.; Tzeng, W. B.; Ng, C. Y., A study of the unimolecular decomposition
of the (C2H4)2
+
complex. The Journal of Chemical Physics 1984, 80, 1482-1489.
28. Venkatesan, T. S.; Mahapatra, S., Exploring the Jahn-Teller and pseudo-Jahn-Teller
conical intersections in the ethane radical cation. J. Chem. Phys. 2005, 123, 114308.
29. Jacovella, U.; Stein, C. J.; Grütter, M.; Freitag, L.; Lauzin, C.; Reiher, M.; Merkt, F.,
Structure and dynamics of the radical cation of ethane arising from the Jahn–Teller and pseudo-
Jahn–Teller effects. Phys. Chem. Chem. Phys. 2018, 20, 1072-1081.
30. Lee, K. L. K.; Rabidoux, S. M.; Stanton, J. F., Cation States of Ethane: HEAT Calculations
and Vibronic Simulations of the Photoelectron Spectrum of Ethane. J. Phys. Chem. A 2016, 120,
7548-7553.
31. Kumar, R. R.; Venkatesan, T. S.; Mahapatra, S., Multistate and multimode vibronic
dynamics: The Jahn–Teller and pseudo-Jahn–Teller effects in the ethane radical cation. Chem.
Phys. 2006, 329, 76-89.
32. Rabalais, J. W.; Katrib, A., Electronic states of C2H6
+
. Molecular Physics 1974, 27, 923-
931.
33. Baker, A. D.; Baker, C.; Brundle, C. R.; Turner, D. W., The electronic structures of
methane, ethane, ethylene and formaldehyde studied by high-resolution molecular photoelectron
spectroscopy. International Journal of Mass Spectrometry and Ion Physics 1968, 1, 285-301.
34. Andrei, H.-S.; Solcà, N.; Dopfer, O., IR Spectrum of the Ethyl Cation: Evidence for the
Nonclassical Structure. Angewandte Chemie (International ed. in English) 2008, 47, 395-7.
35. Wagner, J. P.; McDonaldII, D. C.; Duncan, M. A., Near-infrared Spectroscopy and
Anharmonic Theory of the H2O
+
Ar1,2 Cation Complexes. J. Chem. Phys. 2017, 147, 104302.
36. Ricks, A.; Douberly, G.; Schleyer, P.; Duncan, M., Infrared spectroscopy of protonated
ethylene: The nature of proton binding in the non-classical structure. Chem. Phys. Lett. 2009, 480,
17–20.
37. Davies, J. A.; Yang, S.; Ellis, A. M., Infrared spectra of carbocations and CH4+ in helium.
Phys. Chem. Chem. Phys. 2021, 23, 27449-27459.
100
38. Nizkorodov, S. A.; Maier, J. P.; Bieske, E. J., The infrared spectrum of the N2H+–He ion‐
neutral complex. J. Chem. Phys. 1995, 102, 5570-5571.
39. Meuwly, M.; Nizkorodov, S. A.; Maier, J. P.; Bieske, E. J., Mid‐infrared spectra of He–
HN2
+
and He2–HN2
+
. The Journal of Chemical Physics 1996, 104, 3876-3885.
101
S. Supplementary Information
Figure S1. Ethylene pickup pressure dependence of the signal for C 2H 3
+
, C 2H 4
+
, C 2H 5
+
, and C 2H 6
+
as
measured at M = 27 (a), M = 28 (b), M = 29 (c), and M = 30 (d), respectively. The droplets were ionized
by electron impact in the probe. In b) the baseline signal due to He 7
+
and N 2
+
at zero ethylene pressure was
subtracted. The red curves are fits to the Poisson dependence for the pickup of single molecules.
102
Figure S2. The spectra obtained upon laser irradiation of the ethane doped ionized droplets. Spectra
recorded at M = 30 (black), 58 (red), 59 (blue), 60 (magenta), 88 (dark yellow), 89 (olive), and 90 (orange).
103
Figure S3. The comparison of spectra recorded at M = 29 upon laser irradiation of helium droplets doped
at 4×10
-6
(black trace) and 8×10
-6
mbar (red trace) ethane pick up pressure. The inset shows an pick up
pressure dependence of the laser induced signal for C 2H 5
+
at 3150.3 cm
-1
as measured at M = 29. Curves
are fits of the data points by Poisson probability for the capture of k = 1 (red), k = 2 (blue), and k = 3
(magenta) molecules per droplet.
104
Chapter 6. Infrared spectroscopy of carbocations in helium droplets
6.1 Introduction
The structure of carbocations has been extensively studied as they serve as intermediates
in many chemical reactions and form important constituents of interstellar clouds. Various aspects
of carbocation structures such as stabilization due to hyperconjugation and existence of two
electron three center bridge bonds in hypervalent non-classical carbocations provide interesting
challenges to the understanding of fundamental concepts of chemical bonding and structure.
Carbocations often have multiple structural isomers having similar formation enthalpies. Different
isomeric structures of carbocations have been obtained depending on the local environment, the
precursor and technique used to generate ions. Studies in gas phase free of any external
environment that may influence carbocation's structure are ideal to explore the minimum energy
structures of carbocations. Infrared spectroscopy is a well-established experimental technique
applicable to distinguish the isomers. However, the spectra in gas phase are often complicated by
hot bands. Superfluid Helium droplets with their weak interaction, and low temperature of 0.4 K
are ideal matrix to study the different isomer structures of carbocations.
1, 2
Rapid cooling to
ultracold temperatures in helium droplet may provide the access to different structures
corresponding to both local and global minimum of the potential energy surface. The formation of
different isomeric metastable structures was previously observed with neutral clusters such as
linear HCN clusters,
3
water hexamer clusters
4
etc. in helium droplets.
In this work we show that different isomers of carbocations could be obtained upon
electron impact ionization of helium droplets doped with different neutral precursor molecules.
105
The cations were studied by the laser infrared spectroscopy in the range of C-H stretches. This
work is focused on the carbocations resulting from the fragmentation of transbutene, propene,
allene and n-butane doped droplets. We also study different isomers of carbocations resulting from
ion-molecule reactions of ionized unsaturated hydrocarbons such as ethylene and acetylene in
helium droplets. We show that helium droplets offer flexibility to access different isomers
depending on the precursor used. In this work we also show that different isomers appear for the
ions resulting from ion-molecule reactions involving acetylene and ethylene. This effect likely
relates to a large space of geometries that is spanned during ion-molecular reactions.
6.2 Results and discussion
6.2.1 C2H
+
2700 2800 2900 3000 3100 3200 3300 3400
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Intensity (arb)
wavenumber (cm
-1
)
C
2
H
+
a)
106
Figure 6.1. a) Infrared spectra of
C 2H
+
obtained upon ionization of the acetylene doped helium droplets
upon detection of M=25. b) Mass spectra obtained with laser set at the maxima of three spectral peaks in a)
at 3182.5(black), 3145.1 (red) and 3111.4 cm
-1
(blue) frequency.
Figure 6.1a shows the IR spectra of C2H
+
obtained upon ionization of helium droplets
doped with acetylene. The spectrum has three prominent peaks at 3182.5, 3145.1 and 3111.4 cm
-
1
. Figure 6.1b shows the mass spectra obtained with laser set at the maxima of each of those peaks
as described in chapter 4. It is seen that the excitation at 3182.5 cm
-1
leads to a progression of
peaks M= 25, 29, 33, etc. corresponding to bare C2H
+
ion and clusters containing several He atoms.
In comparison the excitation at 3145.1 cm
-1
shows the two progression of He cluster peak starting
at M= 25 and M=27. We assign the 3145.1 cm
-1
peak to excitation of the complexes of C2H
+
with
hydrogen molecules. Similar effect was observed in our previous study of C 2H2
+
ions obtained
from ethylene precursor molecules. Upon the laser excitation the C2H
+
-H2 may stay intact or
0 10 20 30 40 50 60
0
1000
2000
3000
4000
5000
6000
7000
8000
Intensity (arb.)
m/z
3182.5 cm
-1
3145.1 cm
-1
3111.4 cm
-1
b)
107
fragment giving rise to the two progressions starting at mass 27 and 25, respectively. Henceforth,
the peak at 3111.4 is assigned to the C-H stretch peak of ethynyl cation C2H
+
. C2H radical has
been extensively studied
5-9
because of its reactivity
10
and importance in interstellar medium.
11
However, studies on C2H
+
are relatively scarce. C2H
+
is linear molecule, showing Renner – Teller
effect. It has
3
ground state as determined by calculations
12
and by translational energy
spectroscopy
he bending vibrations of C2H
+
were studied using the velocity imaging of the
fragments upon Coulomb explosion.
14
The first IR spectra of C2H
+
C-C
stretching in the NIR
region was observed in solid argon and neon at 1820.4 and 1832.2 cm
-1
respectively.
15
Here we
report the first IR peak of C-H stretch of C2H
+
at 3111.4 cm
-1
.
6.2.2 C3H2
+
Figure 6.2. a) Infrared spectrum of C 3H 2
+
obtained upon electron impact ionization of allene in helium
droplets b) Cyclic C3H2
+
c) Linear C3H2
+
.
2600 2800 3000 3200 3400 3600
0.0
0.5
1.0
1.5
2.0
2.5
Intenisity
wavenumber (cm
-1
)
allene precursor
C
3
H
2
+
a)
108
Mid IR C-H
stretch
assignment
This Work
in He droplets
Ne tagged IRPD
16
Calculated
intensity
(km/mol)
16
Ab initio
calculations
16
Linear C3H2
+
v3
v2+v4
3205.9
3154.3
3204
3152
271
45
3229
3173
Cyclic C3H2
+
v1
v7
-
-
3139
3116
48.3
136.4
3139
3123
Table 6.1 C-H vibrational frequencies of C 3H 2
+
obtained in helium droplets and measured with Ne tagging.
Both measurements were done with allene precursor. The last two columns show the results of quantum
calculations using coupled cluster methods. All frequencies are in cm
-1
.
Figure 6.2 shows the infrared spectra of C3H2
+
obtained from allene (H2C=C=CH2) doped
helium droplets with peaks at 3154.3 and 3205.9 cm
-1
. C3H2
+
is believed to be an important
reaction intermediate in interstellar clouds as its neutral counterpart C3H2 is detected in diffuse and
dark cloud cores in different isomeric forms.
17
According to calculations C3H2
+
has three stable
isomer structures- ground state cyclopropenylidine ion (c-C3H2
+
), HCCCH
+
linear isomer (l-
C3H2
+
) at 28 kJ/mol higher energy and another CCCH2
+
linear C2v isomer at 182 kJ/mol higher
energy.
18
The first two isomers were identified in the study of ion molecule reaction of HCCC
+
with H2 molecules. The abundance of c-C3H2
+
and l-C3H2
+
in selected ion flow tube (SIFT)
experiments was found to be in a ratio of 20:80 .
19
The vibrational frequencies for the c-C3H2
+
were first reported from photoelectron spectroscopy of c-C3H2.
20
Later Ne-tagged IRPD
measurements in cryogenic ion traps produced by electron impact ionization of allene reported the
109
vibration spectra of both c-C3H2
+
and l-C3H2
+
isomers.
16, 21
Table 6.1 compares the vibrational
frequencies obtained in this work with Ne-tagged IRPD and calculations using coupled cluster
methods.
16
The peaks at 3205.9 cm
-1
and 3154.3 cm
-1
can be assigned to the v3 and v2+v4 vibrations
of l-C3H2
+
based on proximity with frequencies obtained in Ne-tagged IRPD measurements
16
and
calculations. Both our and IRPD tagging measurements used electron impact ionization of allene.
However, in measurements with tagging both c-C3H2
+
and l-C3H2
+
isomers were identified with
an abundance ratio of 60:40 whereas only the l-C3H2
+
isomer is detected in helium droplets. In the
tagging experiments
16
the electron impact takes place in a warm region close to the nozzle, and
the primary products of the electron impact ionization may participate in secondary ion-molecule
reactions or collision induced interconversion of the isomers. It is possible that the l-C3H2
+
isomer
is the primary product of the electron impact dissociative ionization of linear allene precursor. Its
conversion to the more stable cyclic form is associated with a potential barrier, which could not be
surmounted in cold He droplets. Therefore, ionization in He droplets may in general yield more
sharp distribution of the isomers, which structure is determined by the precursor used. We
anticipate that c-C3H2
+
will primarily be produced upon ionization of cyclo-propene or cyclo-
propane, for example. Further investigation into the occurrence of isomers formed with electron
impact ionization of different precursors in helium droplets is required to clarify the relation of the
abundance of the isomers to their precursors.
110
6.2.3 C3H3
+
Figure 6.3. a) Infrared spectra of C 3H 3
+
obtained upon ionization of helium droplets doped with propene
(black trace), ethylene (red trace) and allene (blue trace) b) cyclopropenyl cation c) linear propargyl
cation
Mid IR C-H
stretch
assignment
Precursor (This work in He) Ar -tagged
IRPD
22
Anharmonic
analysis
calculations
23
Allene Propene Ethylene
c- C3H3
+
v4 (e’)
3135.8
3135.8
3132
3134.8
l- C3H3
+
v2 (a1)
v9 (b2)
v1 (a1)
3003.4
3236.4
3236.4
3236.4
3004
3093
3238
2998.7
3071
3239
unassigned 3197.8
3185.2
3145.4
3124.1
3145.4
Table 6.2. Comparison of C-H vibrational frequencies for the two isomers of C 3H 3
+
obtained in helium
droplets with different precursors, with Ar tagging measurements and anharmonic calculations using second
order perturbation theory. All frequencies are in cm
-1
.
2900 3000 3100 3200 3300 3400 3500 3600
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Intenisity
wavenumber (cm
-1
)
Precursors
propene
ethylene
allene
C
3
H
3
+
a)
111
The spectra of some ions such as C3H3
+
have been obtained in this work using different
precursors. Figure 6.3a shows the vibrational spectrum of C3H3
+
from electron impact ionization
of helium droplets doped with propene (H2C=CH-CH3) (black trace), ethylene (C2H4) (red trace)
and allene (C3H4) (blue trace). The spectra from all three precursors shows intense narrow (width
~ 1cm
-1
) IR peak at 3236.4 cm
-1
. Another strong band is seen at 3145.4 cm
-1
from ethylene
precursor. A number of weaker bands are also observed from allene and propene. The frequencies
of the weaker bands listed in Table 6.2 are different for different precursors. The C3H3
+
results
from fragmentation for propene and allene and from ion-molecule reactions with ethylene
precursor as described in Ref
24
. C3H3
+
is one of the fundamental carbocations of importance as an
intermediate in physical organic chemistry and interstellar medium. The structure of the C3H3
+
ion
has been extensively investigated by both theory
25-29
and experiment
22, 30-33
with recognized two
low energy isomers – cyclopropenyl cation (c-C3H3
+
, Fig 1b) and propargyl cation (l-C3H3
+
, Fig
1c) with a linear backbone. The cyclic structure, which is the smallest aromatic cation is more
stable and lower in energy by 25 kcal/mol compared to propargyl cation. The isomers of C3H3
+
cation have been studied using vibrational spectroscopy in superacids,
31
matrix isolation
34
and
infrared photodissociation (IRPD) by tagging with Ar
22
and N2
33
species. Table 6.2 compares the
frequencies obtained in this work with Ar tagged IRPD
22
spectra and theoretical calculations using
vibrational second order pertubation theory and variational methods from Ref
23
. Due to its high
symmetry (D3h) c-C3H3
+
has only one strong IR active C-H band (v4) calculated to be around 3135
cm
-1
whereas the propargyl cation should have three bands (v1,v2,v9). Accordingly we assigne the
band at 3135.8 cm
-1
to c-C3H3
+
. This is in good agreement with 3123 cm
-1
as obtained by Ar
tagging. The spectrum with ethylene precursor shows a strong peak at 3145.4 cm
-1
, which is likely
due to C2H3
+
-He3 clusters. The peak at 3145.4 is a strong vibrational feature from C 2H3
+
as
112
discussed in Chapter 4. In addition, the spectrum from ethylene precursors shows strong peak at
3236.4 cm
-1
which was also the dominant peak from allene and propene. This is in good agreement
with the results of the Ar tagging experiments which have reported the v1 band of propargyl linear
cation at 3238 cm
-1
.
22
Based on the proximity of these results we assign the intense band at 3236.4
cm
-1
with all the precursors to v1 band of the linear cation. Some weak bands in the range of 3000-
3200 cm
-1
may come from l-C3H3
+
isomer or rotational structure of its antisymmetric perpendicular
CH2 band, which unambiguous assignment could not be currently achieved.
Based on these temporary assignments we conclude that both the isomers of C3H3
+
are
present in helium droplets. In case of propene precursor, the peak due to c-C3H3
+
isomer is much
weaker than for l-C3H3
+
and the first is not discernable for the ethylene precursors. This is in
agreement with the conjecture that the ionization of linear precursors yield linear ions, as discussed
in the previous section. In fact some small abandance of the c-C3H3
+
from propene, may stem not
from direct dissociative ionization, but from some secondary reactions involving smaller fragment
ions, such as C2H2
+
. In future, it would be desirable to test this hypothesis by attempting to obtain
a prominent c-C3H3
+
isomer using cyclic precursor such as cyclopropane, cyclopropanol or
cyclopropene
113
6.2.4 C3H4
+
Figure 6.4. a) Infrared spectra of C 3H 4
+
in He droplets obtained with propene (black) and allene (red)
precursors b) Allene c) Propyne ions.
Table 6.3. C-H vibrational frequencies for the two isomers of C 3H 4
+
obtained in helium droplets with
different precursors compared with the frequencies obtain in Ar Matrix and results of harmonic vibrational
calculations using B3PW91/aug-cc-pVTZ. All frequencies are in cm
-1
.
Mid IR C-H
stretch
assignment
Precursor (This
work in He)
Ar
matrix
35
Harmonic
Calculations
35
IR
intensity
35
km/mol Allene Propene
Allene C3H4
+
v5
v8
2949.6
3040.8
2949.6
3040.8
2929
3020.8
3065
3162
100
60
Propyne C3H4
+
v1
v2
v3
3201.8
2942.1
3201.8
2942.1
3214.5
2780.2
3354
3033
2868.6
100
23
90
Unassigned 3145
3170
3221.6
2600 2800 3000 3200 3400 3600
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Intenisity
wavenumber (cm
-1
)
Precursor
propene
Allene
C
3
H
4
+
a)
114
Figure 6.4 shows the IR spectra of C3H4
+
in helium droplets obtained with allene (red trace)
and propene (black trace) precursor. The spectra with allene precursor shows IR peaks at C 3H4
+
2942.1, 2949.7, 3040, 3145, 3170, 3201.8 and 3221.6 cm
-1
. On the other hand, spectrum with
propene precursor has two intense peaks at 2949.7 and 3040.8 cm
-1
, and comparatively weak peaks
at 2942.1, 3145, 3201.8 and 3221.6 cm
-1
. C3H4 has two structures – propyne and allene. Similarly,
for C3H4
+
two isomers of allene and propyne cation exist as depicted in Fig. 6.4 b), c) with ground
state electronic level splitting X
2
E resulting from the Jahn Teller distortion of their neutral
counterparts. Theoretical calculations predict allene cation to be more stable than propyne cation
by 15.4 kcal/mol.
36, 37
Few experimental spectroscopic studies are available for C3H4
+
.
Photoelectron spectra measured few vibrational modes of allene and propyne cations.
38
IR features
of predominantly allene cations were observed in matrix deposition with Ne atoms, where it was
observed that propyne cations isomerized to stable allene isomers.
39
IR spectra of both allene and
propyne cations generated upon UV irradiation were observed in matrix deposition measurements
with argon atoms.
35
In Table 6.3 we temporarily assign the peaks observed to allene and propyne
cations. The intense peaks at 2949.6 and 3040.8 cm
-1
seen with both allene and propene precursor
are assigned to the vibrational modes of the stable allene cations. The weaker peaks at 3201.8 and
2942.1 cm
-1
are assigned to the propyne cation. The relative intensity of the peaks from propyne
cation and allene changes from allene precursor to propene precursor indicating change in the
composition of isomers generated inside droplets with change in precursor. It is likely that more
intense peaks from propyne cations could be obtained from propyne precursors.
115
6.2.5 C3H5
+
Figure 6.5. a) IR spectrum of C 3H 5
+
in helium droplets obtained using different precursors: propene (blue
trace), ethylene (black trace), n-butane (pink trace) and allene (red trace) b) allyl cation c) 2-propenyl cation.
Figure 6.5a shows the vibrational spectrum of C 3H5
+
in helium droplets obtained with
propene (blue trace), ethylene (black trace), n-butane (pink trace) and allene (red trace) precursors.
The spectrum shows intense narrow IR peak at 2936.8 cm
-1
with all four precursors. Weaker peaks
are found at 2819.4, 2996.4 and 3010 cm
-1
. Very weak peak is observed at 3120 cm
-1
, which was
only found with allene and propene precursors. Two isomers of C3H5
+
- 2-propenyl cation (fig6.5c)
and allyl cation (fig6.5.b) have been extensively studied.
40-42
The resonance stabilized allyl cation
2600 2800 3000 3200 3400 3600
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Intensity
wavenumber (cm
-1
)
Precursor
ethylene
allene
propylene
n-butane
a)
116
is a simple model for investigating electron delocalization in conjugated systems with equal
calculated C-C bond lengths of bond order 1.44 and CCC bond angle 117. 2
O
-propenyl cation. On
the other hand 2-propenyl cation has linear carbon chain of the propenyl cation with sp hybridized
center carbon has calculated energy higher than allyl cation by 8 kcal/mol.
43
The spectra from all
precursors have an intense peak at 2936 cm
-1
. Calculations show that 2-propenyl cation has three
infrared active C-H modes.
42
Argon tagged IRPD experiments reported the IR bands of the 2-
propenyl cation at 2800, 2936 and 3005 cm
-1
.
42
Upon comparison with the results of the argon
tagged measurements and ab initio calculations as shown in Table 6.4 we assign the peaks at 2819
cm
-1
and 2936.8 cm
-1
to methyl C-H stretch and ethylenic CH2 symmetric stretch respectively. The
spectrum has two peaks within the ethylenic asymmetric stretch region at 2996.4 and 3010 cm
-1
whereas only one peak was observed at 3005 cm
-1
with Ar-tagged IRPD measurements. The
relative intensities of the different C-H peaks arising from 2- propenyl cation are also different
from that observed in Ar-tagged measurements. In He droplets the spectrum shows an intense peak
at 2936.8 cm
-1
and several other weak peaks. Based on the intensity of the peaks with different
precursors it appears that 2-propenyl cation isomer is the major isomer of C3H5
+
obtained in helium
droplets. The C2v allyl cation is expected to have one IR active C-H vibration rising from the in
phase asymmetric stretch of two CH2 moeties. Buzek et.al
44
first reported the infrared spectrum of
the allyl cation in cryogenic SbF5 matrix with asymmetric CH2 stretch at 3117 cm
-1
. Based on the
proximity of the frequencies we assign the weak peak at 3120 cm
-1
observed with allene and
propene precursors to the allyl cation isomer. This peak was only observed with allene and
propene precursors. This is also different from the results of the Ar tagged measurements where
both isomers were observed with similar abundance. It was also observed that the relative
proportion of the isomers is different for different precursors as discussed in Ref
42
. The peak at
117
3157.4 cm
-1
observed with propene precursor seems to be coming from fragmentation of C3H6
+
as
IR peak is observed at same frequency for C3H6
+
spectra in section 6.2.6.
Mid IR C-H stretch
assignment
Precursor (This work) Ar -tagged
IRPD
42
IR intensity
(km/mol)
42
Allene Propene Ethylene n-butane
2-propenyl C3H5
+
Methyl C-H stretch
Ethylenic symmetric
CH2 stretch
-
Ethylenic asym. CH2
stretch
2819.3
2936.8
2996.4
3010
2819.3
2936.8
2996.4
3010
2819.3
2936.8
3010
2819.3
2936.8
2996.4
3010
2800
2936
3005
168
232
145
Allyl C3H5
+
In phase asymmetric
stretch
3120
3120
3112
21
Table 6.4. C-H vibrational frequencies for the two isomers of C 3H 5
+
obtained in helium droplets with
different precursors, with Ar tagging and anharmonic calculations using second order perturbation theory.
All frequencies are in cm
-1
.
6.2.6. C3H6
+
Figure 6.6 shows the IR spectra of C3H6
+
cation in helium droplets obtained with propene
(C3H6) precursor molecules. The spectrum shows two intense bands at 3238.3 and 3158 cm
-1
with
less intense spectral peaks at 3093, 2950.2, 2933.4 and 2775.8 cm
-1
. IR and Raman spectra of
neutral propene C3H6 has been studied in 200 - 4000cm
-1
to understand the hindered methyl
rotation around carbon double bond.
45-48
Vibrational frequencies of the low energy modes have
been studied using photoelectron spectra.
49, 50
C3H6 has eclipsed and staggered conformations
118
involving relative angles of the methyl CH3 and methylene CH2 groups. It has been found that the
barrier to rotation is reduced in C3H6
+
cation (50-100 cm
-1
) compared to the neutrals (1000 cm
-1
).
Torsional analysis of these methyl and methylene groups has been done using photoelectron
spectra of the methylene v20 and methyl v21 torsional modes below 2000 cm
-1
.
51, 52
Here we report
the vibrational spectra of C3H6
+
(X
+
2
A” electronic ground state) cations by using propyne neutral
precursor for the first time. The DFT calculations in Ref
49
predict two intense v1, v2 C-H bands of
C3H6
+
at 3246 and 3160 cm
-1
. Based on proximity of the frequencies we assign the two strong
peaks at 3238.3 and 3158 cm
-1
to v1 and v2 modes
of C3H6
+
. Further work is needed to assign the
complete spectrum.
Figure 6.6. Infrared spectra of C 3H 6
+
obtained in He droplets with propene precursors.
2700 2800 2900 3000 3100 3200 3300 3400
0.4
0.6
0.8
Intensity
Wavenumber (cm
-1
)
C
3
H
6
+
119
6.2.7 C4H2
+
Figure
6.7. a) Infrared spectrum of C 4H 2
+
obtained upon ionization of acetylene doped helium droplets b-
e) Lowest energy isomers of C 4H 2
+
Figure 6.7 shows infrared spectrum of C4H2
+
in helium droplets doped with acetylene
molecules as well as its lowest energy isomers. The spectrum has two strong peaks at 3230.2 and
3140.4 cm
-1
and weak peaks at 3156, 3204.1 and 3234.2 cm
-1
. Diacetylene ions (Fig. 6.7 b) have
been studied since they are considered to be important intermediates in the ion-molecule reactions
2900 3000 3100 3200 3300
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Intensity (arb)
wavenumber (cm
-1
)
C
4
H
2
+
a)
120
leading to formation of benzene ions detected in interstellar system.
53
The electronic spectrum of
A
2
u−X
g transitions of the linear diacetylene cation has been extensively studied using
emission spectra in electric discharges,
54
by photoelectron spectroscopy
55
and in low temperature
Ne matrices.
56
The IR absorption spectra of the lowest energy linear C4H2
+
(X
g) conformer was
reported in Argon matrix with the frequency of the C-H stretching mode at 3201.6 cm
-1
and of the
stretching involving the triple C-C bond at 1827.9 cm
-1
.
57
Based on proximity of the frequency we
assign the band at 3230.2 cm
-1
to diacetylene cations. This assignment is also consistent with the
results of theoretical calculations which have shown that the linear C4H2
+
(figure 6.7b) is the lowest
energy isomer.
58, 59
The presence of rather intense band at 3140.4 cm
-1
in the spectrum indicates
the presence of some other isomer. The frequency of the additional band is broadly with in the sp
2
,
sp C-H stretching range. Figure 6.7 b-d shows four lowest energy isomers of C4H2
+
. Isomer in
Fig 6.7c is higher in energy than the linear isomer by 82 kJ/mol and isomers 6.7d, e are almost
equal in energy with ~160 kJ/mol above the lowest energy isomer.
59
Harmonic vibrational
frequencies and infrared intensities of the vibrational modes of these isomers were calculated at
the CCSD level.
59
The strong IR intensities (IRI) for C-H stretching were computed to be at 3376
(252 km/mol) and 3343 (278 km/mol) cm
-1
for the linear and four membered ring isomer
respectively.
59
Based on these calculations and the IR data from Ar matrix
57
we tentatively assign
the intense peaks at 3230.2 and 3140.4 cm
-1
to the C-H stretching modes of linear and four
membered ring isomer of C4H2
+
respectively. The weak peaks might be arising from the higher
energy isomers in Figure 6.7d and e or some combination bands.
121
6.2.8 C4H3
+
and C4H4
+
Figure
6.8. a) Infrared spectra of C 4H 3
+
and C 4H 3
+
obtained upon ionization of acetylene doped helium
droplets b-d) Lowest energy isomers of C 4H 3
+
e-h) Lowest energy isomers of C 4H 4
+
Figure 6.8a shows the infrared spectra of C4H3
+
and C4H4
+
from acetylene doped helium
droplets. The spectrum of C4H3
+
(black trace)
shows four intense peaks at 2944, 3008.5, 3030.3 and
3244.5 cm
-1
with weak peaks at 2952.1, 3136.2 and 3145.5 cm
-1
. Fig 6.8 b-d shows the low energy
2900 3000 3100 3200 3300
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Intensity
wavenumber (cm
-1
)
C
4
H
3
+
C
4
H
4
+
a)
122
isomers of C4H3
+
according to theoretical calculations.
60
The peaks at 2944, 3008.5, 3030.3 cm
-1
seem to lie in the sp
3
C-H stretching range whereas the peak at 3244.5 is in sp C-H stretching range
which likely stems from structure 6.8.b. Because more than three peaks in the C-H stretching
region are seen, it is likely that several isomers of C4H3
+
are present. The formation of C4H3
+
from
acetylene clusters has been studied using mass spectrometry.
61, 62
However, presented here is the
first reported experimental IR spectrum of C4H3
+
.
The spectrum of C4H4
+
(red trace)
shows five intense peaks at 2993.2, 3012.5, 3115.7,
3134.5, 3250.8 cm
-1
with weak peaks at 2943.8, 3078.1, 3151.6, 3164.7 and 3219.9 cm
-1
. The
peaks at 3012.5 and 3250.8 appear close to the strong peaks for C4H3
+
but slightly shifted. Cyclic
Isomers of neutral C4H4 – triafulvene, cyclobutadiene and tetrahedrane have been extensively
studied due to their aromatic/antiaromatic properties.
63
Linear vinylacetylene and butatriene are
other possible isomers of C4H4.
64
Similar isomers are assumed to occur in the case of the radical
cation C4H4
+.
.
65
The ionization of neutral tetrahedrane C4H4 precursors was calculated to rearrange
to cyclobutadiene C4H4
+.
.
66
Different structures of C4H4
+.
has been detected in mass spectrometer
studies as a fragmentation product of many organic molecules such as benzene, pyridine quinones
etc.
67-69
Quantitative estimation of isomer composition from different precursors was done using
techniques such as neutralization-reionization mass spectroscopy
67, 70
or by studying ion-molecule
reactions.
71
Structural information on the cyclobutadiene (CB, fig 6.8h) and
methylenecyclopropene (MCP, fig 6.8g) cations is available from photoelectron spectra of
cyclobutadiene
72
and methylenecyclopropane.
73
CB structure of C4H4
+.
was observed in ion-
molecule reactions of alkylated acetylene and its radical cation.
74
The reaction of acetylene and
acetylene cation leading to formation of different isomers 6.8e-h has been studied theoretically
using coupled cluster and density functional methods.
75
To the best of our knowledge reported in
123
Figure 6.8 is the first infrared spectrum obtained for C4H4
+.
. The spectrum shows ten IR peaks in
the C-H stretch region covering sp, sp
2
and sp
3
C-H regions indicating the presence of at least three
isomers of C4H4
+.
. Individual alignment of the peaks to their respective isomers requires further
work.
6.2.9 C4H5
+
Figure 6.9a shows the IR spectra of C4H5
+
obtained in helium droplets from ethylene (black
trace) and acetylene (red trace) precursors. The spectra from both precursors show two strong
peaks at 3208 and 2960.5 cm
-1
. Weaker peaks are found at 3254.8, 3177 and 3037.8 cm
-1
. C4H5
+
like other carbocations is of astronomical interest and has been detected in Titan’s atmosphere.
76
The electronic spectrum of linear 1-butyn-3yl and 2-butyn-3-yl isomers C4H5
+
has been studied
extensively using photoelectron spectroscopy.
77-79
Various isomers of C4H5
+
have been studied in
gas phase and in solutions and characterized using NMR.
80-83
The available information on the
infrared spectrum of C4H5
+
is limited to the proton bound acetylene dimer , for which IRPD
experiments with Ar tagging found a doublet peak at 3129 and 3158 cm
-1
.
84
The spectra in figure
6.9 likely correspond to a covalently bound form of C4H5
+
cation which is obtained for the first
time. It is unlikely that the spectra in Fig. 6.9 stem from protonated acetylene dimer C4H5
+
because
of its large difference from that obtained in the Argon tagged IRPD measurements.
84
The intense
peak at 2960.5 cm
-1
is most likely occurring from ethylenic C-H stretch as the frequency is close
to ethylenic stretch observed with C3H4
+
(2949.6 cm
-1
) and C3H5
+
(2936.8 cm
-1
). The intense peak
at 3208 cm
-1
most likely occurs from sp C-H stretch. The relative intensity of the two peaks varies
with change of precursor from acetylene to ethylene which implies both the peaks may arise from
different isomers of C4H5
+
. Figure 6.9 b-f shows the low energy isomers of C4H5
+
according to
124
theoretical calculations.
85
Tentatively the peak at 2960.5cm
-1
can be assigned to linear structures
in figure 6.9 d or e and the peak at 3208 cm
-1
can be assigned to structure in figure 6.9 f.
Figure
6.9. a) Infrared spectra of C 4H 5
+
obtained upon ionization of in acetylene and ethylene doped helium
droplets b-e) Lowest energy isomers of C 4H 5
+
2900 3000 3100 3200 3300
0.0
0.5
1.0
1.5
2.0
2.5
Intenisity
wavenumber (cm
-1
)
Precursor
Ethylene
Acetylene
a)
125
6.2.10 C4H7
+
Figure
6.10. a) Infrared spectra of C 4H 7
+
upon ionization of helium droplets doped with acetylene and
transbutene. b-e) Lowest energy isomers of C 4H 7
+
Figure 6.10a shows the infrared spectra of C4H7
+
in helium droplets doped with ethylene
(red trace) and transbutene (CH3-CH=CH-CH3) (black trace). Ethylene doped droplets (Fig.6.10a
red trace) show narrow peaks of ~ 1cm
-1
width at 3242.2, 3209.2, 3145.5 and 3124.4 cm
-1
, broad
peaks of width 20 and 50 cm
-1
at 2916.9 and 2751.6 cm
-1
respectively. Transbutene doped droplets
(Fig.6.10a black trace) show narrow peaks at 3242.2 and 3124.4 cm
-1
at the same frequency
observed with ethylene doped droplets and new peaks at 2863.2 and 2943.8 cm
-1
. The structure
and dynamics of C4H7
+
has been extensively interrogated since the solvolysis studies of cyclobutyl,
cyclopropylcarbinyl and homoallyl derivatives by J.D.Roberts.
86
Based on various theoretical
87
and experimental studies using NMR,
88
mass spectrometry,
89
IR spectroscopy in superacid
matrices
90
and photoelectron spectroscopy
91
it has been proposed that C4H7
+
exists as rapidly
interconverting structure between bicyclobutonium(Fig 6.11.c) and cyclopropylcarbinyl cations
2600 2700 2800 2900 3000 3100 3200 3300 3400
0.00
0.05
0.10
0.15
0.20
0.25
Intensity (arb)
wavenumber (cm
-1
)
Precursor
Ethylene
Transbutene
a)
126
(Fig 6.11.b).
92
However, calculations predict 1-methylallyl as the most stable isomer (Fig 6.11.d).
The only IR spectra of C4H7
+
bicyclobutonium and cyclopropylcarbinyl cations were reported in
SbF5 matrices at 180 K with peaks at 3138, 3013, 2990, 2947 and 2859 cm
-1
which rearranged to
1-methylallyl cation at 230 K, with new peaks appear at 3085, 2937 cm
-1
.
90
In the sp
2
and sp C-H
stretching region, the spectra of C4H7
+
in helium droplets in figure 6.10 a) show four and two
sharp IR peaks with ethylene and transbutene precursor, respectively. This behavior indicates the
presence of more than one isomer of C4H7
+
in the case of ethylene precursor. The different isomers
likely form during the formation of covalently bound C4H7
+
ions upon ionization of dimers which
form in the droplets upon doping with two ethylene molecules. In particular, ethylene molecules
in the dimer can have different mutual orientation with propensity of forming different isomers of
C4H7
+
upon the ionization. The two narrow peaks at 2863.2 and 2943.8 cm
-1
with transbutene
precursor most likely come from methyl C-H stretch or ethylenic symmetric and asymmetric CH2
stretch based on the frequencies observed with C3H4
+
and
C3H5
+
. The broad intense peaks at 2916.9
and 2751.6 cm
-1
of C4H7
+
with ethylene doped droplets are unusual and warrant some future
studies. Because of the large width and high intensity of the peaks, the bands must have very high
infrared intensity. The peaks may for example correspond to proton bound dimer of ethylene and
acetylene molecules. The individual assignment of the peaks to their respective isomers in Fig 6.10
b-e is beyond the scope of the current thesis and further analysis supported by quantum calculations
is required.
127
6.3 Conclusion
In this chapter we have studied the IR spectra of different isomers of carbocations resulting
from electron impact ionization of different precursors - transbutene, propene, allene, n-butane,
acetylene and ethylene in helium droplets. We found that the ionization of linear precursors leads
to linear isomers of carbo-ions as major products as opposed to their thermodynamically stable
cyclic structures. For many ions such as C3H2
+
, C3H4
+
, C3H3
+
, C4H2
+
we were able to identify
several isomers. For some carbocations such as C3H4
+
the relative intensity of the IR peaks from
different isomers changed with precursor indicating strong dependence of the isomer generated on
the precursor used. For ions resulting from secondary reactions of the ionized and neutral
precursors more isomers were observed due to the steric flexibility of the reactants. This shows
that helium droplets can be used to study the various structures of carbocations not just the
energetically stable structures. In addition, the IR spectra of the C2H
+
, C4H3
+
, C4H4
+
, C4H5
+
, C4H7
+
cations are reported for the first time in this work. In future, it would be interesting to expand the
variety of the available isomers of the ions by using cyclic precursors, which may result in the
formation of the cyclic ions.
6.4 References
1. Nauta, K.; Miller, R. E., Solvent mediated vibrational relaxation: Superfluid helium droplet
spectroscopy of HCN dimer. J. Chem. Phys. 1999, 111, 3426-3433.
2. Pörtner, N.; Vilesov, A. F.; Havenith, M., The formation of heterogeneous van der Waals
complexes in helium droplets. Chem. Phys. Lett. 2001, 343, 281-288.
3. Nauta, K.; Miller, R. E., Nonequilibrium self-assembly of long chains of polar molecules
in superfluid helium. Science 1999, 283, 1895-1897.
128
4. Nauta, K.; Miller, R. E., Formation of cyclic water hexamer in liquid helium: The smallest
piece of ice. Science 2000, 287, 293-295.
5. Gans, B.; Garcia, G. A.; Holzmeier, F.; Krüger, J.; Röder, A.; Lopes, A.; Fittschen, C.;
Loison, J.-C.; Alcaraz, C., Communication: On the first ionization threshold of the C2H radical. J.
Chem. Phys. 2017, 146, 011101.
6. Cool, T. A.; Goodwin, P. M., Observation of an electronic state of C 2H near 9 eV by
resonance ionization spectroscopy. J. Chem. Phys. 1991, 94, 6978-6988.
7. Fahr, A., Ultraviolet absorption spectrum and cross-sections of ethynyl (C2H) radicals. J.
Mol. Spectrosc. 2003, 217, 249-254.
8. Wu, Y.-J.; Cheng, B.-M., Infrared absorption spectra of ethynyl radicals isolated in solid
Ne: Identification of the fundamental C–H stretching mode. Chem. Phys. Lett. 2008, 461, 53-57.
9. Sharp-Williams, E. N.; Roberts, M. A.; Nesbitt, D. J., High resolution slit-jet infrared
spectroscopy of ethynyl radical: 2Π–2Σ
+
vibronic bands with sub-Doppler resolution. J. Chem.
Phys. 2011, 134, 064314.
10. Soorkia, S.; Trevitt, A. J.; Selby, T. M.; Osborn, D. L.; Taatjes, C. A.; Wilson, K. R.; Leone,
S. R., Reaction of the C2H Radical with 1-Butyne (C4H6): Low-Temperature Kinetics and Isomer-
Specific Product Detection. J. Phys. Chem. A 2010, 114, 3340-3354.
11. Ziurys, L. M.; Saykally, R. J.; Plambeck, R. L.; Erickson, N. R., Detection of the N = 3-2
transition of CCH in Orion and determination of the molecular rotational constants. Astrophys. J.
1982, 254, 94-99.
12. Hashimoto, K.; Iwata, S.; Osamura, Y., An MCSCF study of the low-lying states of C2H
+
.
Chem. Phys. Lett. 1990, 174, 649-654.
13. O'Keefe, A.; Derai, R.; Bowers, M. T., The first experimental observation of electronic
transitions in C2
+
and C2D
+
. Chem. Phys. 1984, 91, 161-166.
14. Zajfman, D.; Vager, Z.; Naaman, R.; Mitchell, R. E.; Kanter, E. P.; Graber, T.; Belkacem,
A., The structures of C2H
+
and C2H2
+
as measured by Coulomb explosion imaging. J. Chem. Phys.
1991, 94, 6377-6387.
129
15. Andrews, L.; Kushto, G. P.; Zhou, M.; Willson, S. P.; Souter, P. F., Infrared spectrum of
CCH+ in solid argon and neon. J. Chem. Phys. 1999, 110, 4457-4466.
16. Brünken, S.; Lipparini, F.; Stoffels, A.; Jusko, P.; Redlich, B.; Gauss, J.; Schlemmer, S.,
Gas-Phase Vibrational Spectroscopy of the Hydrocarbon Cations l-C3H
+
, HC3H
+
, and c-C3H2
+
:
Structures, Isomers, and the Influence of Ne-Tagging. J. Phys. Chem. A 2019, 123, 8053-8062.
17. Turner, B. E.; Herbst, E.; Terzieva, R., The Physics and Chemistry of Small Translucent
Molecular Clouds. XIII. The Basic Hydrocarbon Chemistry. The Astrophysical Journal
Supplement Series 2000, 126, 427-460.
18. Wong, M. W.; Radom, L., Thermochemistry and ion-molecule reactions of isomeric C3H2
.+
cations. J. Am. Chem. Soc. 1993, 115, 1507-1514.
19. Smith, D.; Adams, N. G., Cyclic and linear isomers of C3H2
+
and C3H3
+
: The C3H
+
+ H2
reaction. Int. J. Mass Spectrom. Ion Processes 1987, 76, 307-317.
20. Hemberger, P.; Noller, B.; Steinbauer, M.; Fischer, I.; Alcaraz, C.; Cunha de Miranda, B.
K.; Garcia, G. A.; Soldi-Lose, H., Threshold Photoelectron Spectroscopy of Cyclopropenylidene,
Chlorocyclopropenylidene, and Their Deuterated Isotopomeres. J. Phys. Chem. A 2010, 114,
11269-11276.
21. Asvany, O.; Markus, C. R.; Salomon, T.; Schmid, P. C.; Banhatti, S.; Brünken, S.;
Lipparini, F.; Gauss, J.; Schlemmer, S., High-resolution rovibrational spectroscopy of c-C3H2
+
:
The ν7 C–H antisymmetric stretching band. J. Mol. Struct. 2020, 1214, 128023.
22. Ricks, A. M.; Douberly, G. E.; Schleyer, P. v. R.; Duncan, M. A., Communications:
Infrared spectroscopy of gas phase C3H3
+
ions: The cyclopropenyl and propargyl cations. J. Chem.
Phys. 2010, 132, 051101.
23. Huang, X.; Taylor, P. R.; Lee, T. J., Highly Accurate Quartic Force Fields, Vibrational
Frequencies, and Spectroscopic Constants for Cyclic and Linear C 3H3
+
. J. Phys. Chem. A 2011,
115, 5005-5016.
24. Erukala, S.; Feinberg, A.; Singh, A.; Vilesov, A. F., Infrared spectroscopy of carbocations
upon electron ionization of ethylene in helium nanodroplets. J. Chem. Phys. 2021, 155, 084306.
130
25. Radom, L.; Hariharan, P. C.; Pople, J. A.; Schleyer, P. v. R., Molecular orbital theory of
the electronic structure of organic compounds. XXII. Structures and stabilities of C3H3
+
and C3H
+
cations. J. Am. Chem. Soc. 1976, 98, 10-14.
26. Raghavachari, K.; Whiteside, R. A.; Pople, J. A.; Schleyer, P. V. R., Molecular orbital
theory of the electronic structure of organic molecules. 40. Structures and energies of C 1-C3
carbocations including effects of electron correlation. J. Am. Chem. Soc. 1981, 103, 5649-5657.
27. Cameron, A.; Leszczynski, J.; Zerner, M. C.; Weiner, B., Structure and properties of C3H3
+
cations. J. Phys. Chem. 1989, 93, 139-144.
28. Glukhovtsev, M. N.; Laiter, S.; Pross, A., Thermochemical Assessment of the Aromatic
and Antiaromatic Characters of the Cyclopropenyl Cation, Cyclopropenyl Anion, and
Cyclopropenyl Radical: A High-Level Computational Study. J. Phys. Chem. 1996, 100, 17801-
17806.
29. Jursic, B. S., Computational study of the singlet cyclopropenyl and triplet cyclopropynyl
cations aromatic stabilization with Petersson's complete basis set ab initio approach. Journal of
Molecular Structure: THEOCHEM 1999, 491, 193-203.
30. Craig, N. C.; Pranata, J.; Reinganum, S. J.; Sprague, J. R.; Stevens, P. S., Vibrational
spectra of cyclopropenyl cations (C3H3
+
, C3D3
+
, C3H2D
+
, and C3D2H
+
) and force constants for this
ion system. J. Am. Chem. Soc. 1986, 108, 4378-4386.
31. Breslow, R.; Groves, J. T., Cyclopropenyl cation. Synthesis and characterization. J. Am.
Chem. Soc. 1970, 92, 984-987.
32. Gilbert, T.; Pfab, R.; Fischer, I.; Chen, P., The zero kinetic energy photoelectron spectrum
of the propargyl radical, C3H3. J. Chem. Phys. 2000, 112, 2575-2578.
33. Dopfer, O.; Roth, D.; Maier, J. P., Infrared Spectra of C3H3
+
-N2 Dimers: Identification of
Proton-Bound c-C3H3
+
-N2 and H2CCCH
+
-N2 Isomers. J. Am. Chem. Soc. 2002, 124, 494-502.
34. Wyss, M.; Riaplov, E.; Maier, J. P., Electronic and infrared spectra of H2C3H
+
and cyclic
C3H3
+
in neon matrices. J. Chem. Phys. 2001, 114, 10355-10361.
35. Liu, M.-C.; Chen, S.-C.; Chin, C.-H.; Huang, T.-P.; Chen, H.-F.; Wu, Y.-J.,
Photoisomerization and Infrared Spectra of Allene and Propyne Cations in Solid Argon. J. Phys.
Chem. Lett 2015, 6, 3185-3189.
131
36. Frenking, G.; Schwarz, H., Ab initio molecular orbital calculations on the interconversion
of allene and propyne cation radicals and the mechanism for hydrogen loss from C 3H4
+·
.
International Journal of Mass Spectrometry and Ion Physics 1983, 52, 131-138.
37. van der Hart, W. J., Ab initio molecular orbital calculations on 1,2-hydrogen shifts in the
ethene, allene and propyne radical cations. Int. J. Mass Spectrom. Ion Processes 1995, 151, 27-34.
38. Ho, G. H.; Lin, M. S.; Wang, Y. L.; Chang, T. W., Photoabsorption and photoionization of
propyne. J. Chem. Phys. 1998, 109, 5868-5879.
39. Forney, D.; Jacox, M. E.; Lugez, C. L.; Thompson, W. E., Matrix isolation study of the
interaction of excited neon atoms with allene and propyne: Infrared spectra of H2CCCH2
+
and
H2CCCH
−
. J. Chem. Phys. 2001, 115, 8418-8430.
40. Radom, L.; Hariharan, P. C.; Pople, J. A.; Schleyer, P. V. R., Molecular orbital theory of
the electronic structure of organic compounds. XIX. Geometries and energies of C3H5 cations.
Energy relations among allyl, vinyl, and cyclopropyl cations. J. Am. Chem. Soc. 1973, 95, 6531-
6544.
41. Reindl, B.; Clark, T.; Schleyer, P. v. R., Empirical force field and ab initio calculations on
allyl cations. J. Comput. Chem. 1997, 18, 533-551.
42. Douberly, G. E.; Ricks, A. M.; Schleyer, P. v. R.; Duncan, M. A., Infrared spectroscopy of
gas phase C3H5
+
: The allyl and 2-propenyl cations. J. Chem. Phys. 2008, 128, 021102.
43. Fairley, D. A.; Milligan, D. B.; Wheadon, L. M.; Freeman, C. G.; Maclagan, R. G. A. R.;
McEwan, M. J., Flow tube and theoretical study of proton transfer reactions of C 3H5
+
ions. Int. J.
Mass spectrom. 1999, 185-187, 253-261.
44. Buzek, P.; von R. Schleyer, P.; Vančik, H.; Mihalic, Z.; Gauss, J., Generation of the Parent
Allyl Cation in a Superacid Cryogenic Matrix. Angewandte Chemie International Edition in
English 1994, 33, 448-451.
45. Fateley, W. G.; Miller, F. A., Torsional frequencies in the far infrared—III: The form of
the potential curve for hindered internal rotation of a methyl group. Spectrochim. Acta 1963, 19,
611-628.
132
46. Möller, K. D.; Meo, A. R. D.; Smith, D. R.; London, L. H., Far‐Infrared Torsional
Vibration Spectra of One‐, Two‐, and Three‐(CX3) Top Molecules. J. Chem. Phys. 1967, 47, 2609-
2616.
47. Silvi, B.; Labarbe, P.; Perchard, J. P., Spectres de vibration et coordonnées normales de
quatre espèces isotopiques de propène. Spectrochimica Acta Part A: Molecular Spectroscopy
1973, 29, 263-276.
48. Durig, J. R.; Guirgis, G. A.; Bell, S., Torsional spectrum and ab initio calculations for
propene. J. Phys. Chem. 1989, 93, 3487-3491.
49. Burrill, A. B.; Johnson, P. M., Torsional analyses of trans-2-butene and propene cations:
A comparative investigation of two prototypical ions with different degrees of symmetry. J. Chem.
Phys. 2001, 115, 133-138.
50. Vasilatou, K.; Merkt, F., Torsional vibrational structure of the propene radical cation
studied by high-resolution photoelectron spectroscopy. J. Chem. Phys. 2011, 135, 124310.
51. Lee, M.; Kim, M. S., One-photon mass-analyzed threshold ionization spectroscopy of 2-
bromopropene (2-C3H5Br): Analysis of vibration and internal rotation in the cation. J. Chem. Phys.
2003, 119, 12351-12359.
52. Bae, Y. J.; Lee, M.; Kim, M. S., One-photon mass-analyzed threshold ionization
spectroscopy of 2-chloropropene (2-C3H5Cl) and its vibrational assignment based on the density-
functional theory calculations. J. Chem. Phys. 2005, 123, 044306.
53. Vuitton, V.; Yelle, R. V.; Cui, J., Formation and distribution of benzene on Titan. Journal
of Geophysical Research (Planets) 2008, 113, E05007.
54. Callomon, J. H., AN EMISSION SPECTRUM OF THE DIACETYLENE ION: A STUDY
OF SCHÜLER'S "T" SPECTRUM UNDER HIGH RESOLUTION. Can. J. Phy 1956, 34, 1046-
1074.
55. Baker, C.; Turner, D. W., High Resolution Molecular Photelectron Spectroscopy. III.
Acetylenes and Aza-Acetylenes. Proceedings of the Royal Society of London. Series A,
Mathematical and Physical Sciences 1968, 308, 19-37.
56. Bondybey, V. E.; English, J. H., Electronic spectrum of the diacetylene radical cation in
solid rare gases. J. Chem. Phys. 1979, 71, 777-782.
133
57. Szczepanski, J.; Wang, H.; Jones, B.; Arrington, C. A.; Vala, M. T., Infrared absorption
spectroscopy of diacetylene ions trapped in solid argon. Phys. Chem. Chem. Phys. 2005, 7, 738-
742.
58. Zhao, Y.; Wan, S.-q.; Liu, H.-l.; Huang, X.-r.; Sun, C.-c., Theoretical studies on structures
and stabilities of C4H2
+
isomers. Chemical Research in Chinese Universities 2013, 29, 150-153.
59. Gronowski, M.; Kołos, R.; Krełowski, J., A theoretical study on structure and spectroscopy
of C4H2
+
isomers. Chem. Phys. Lett. 2013, 582, 56-59.
60. Peverati, R.; Bera, P. P.; Lee, T. J.; Head-Gordon, M., Insights into Hydrocarbon Chain
and Aromatic Ring Formation in the Interstellar Medium: Computational Study if the Isomers of
C4H3
+
, C6H3
+
AND C6H5
+
and their formation Pathways Astrophys. J. 2016, 830, 128.
61. Gobeli, D. A.; Simon, J. D.; El-Sayed, M. A., Dynamics of multiphoton ionization-
dissociation of 2,4-hexadiyne by the two-color picosecond pump-pump mass spectrometric
technique: formation of C6H5
+
, C4H4
+
, and C4H3
+
ions. J. Phys. Chem. 1984, 88, 3949-3951.
62. Petrie, S.; Knight, J. S.; Freeman, C. G.; MacLagan, R. G. A. R.; McEwan, M. J.; Sudkeaw,
P., The proton affinity and selected ion/molecule reactions of diacetylene. Int. J. Mass Spectrom.
Ion Processes 1991, 105, 43-54.
63. Jursic, B. S., Theoretical study of structural properties, infrared spectra, and energetic
properties of C4H4 isomers. Journal of Molecular Structure: THEOCHEM 2000, 507, 185-192.
64. Cremer, D.; Kraka, E.; Joo, H.; Stearns, J. A.; Zwier, T. S., Exploration of the potential
energy surface of C4H4 for rearrangement and decomposition reactions of vinylacetylene: A
computational study. Part I. Phys. Chem. Chem. Phys. 2006, 8, 5304-5316.
65. Roeselová, M.; Bally, T.; Jungwirth, P.; Ársky, P., Cyclobutadiene radical cation. An ab
initio study of the Jahn-Teller surface. Chem. Phys. Lett. 1995, 234, 395-404.
66. Hrouda, V.; Bally, T.; Čársky, P.; Jungwirth, P., The C4H4
•+
Potential Energy Surface. 2.
The Jahn−Teller Stabilization of Ionized Tetrahedrane and Its Rearrangement to Cyclobutadiene
Radical Cation,1. J. Phys. Chem. A 1997, 101, 3918-3924.
67. Zhang, M. Y.; Carpenter, B. K.; McLafferty, F. W., Gas-phase formation of four isomeric
C4H4
.+
ions. Ionic isomer quantitation with neutralization-reionization mass spectrometry. J. Am.
Chem. Soc. 1991, 113, 9499-9503.
134
68. Bakhtiar, R.; Drader, J. J.; Jacobson, D. B., Direct formation of cyclobutadiene radical
cation from cis-3,4-dichlorocyclobutene. Org. Mass Spectrom. 1993, 28, 797-799.
69. Maier, G., Tetrahedrane and Cyclobutadiene. Angewandte Chemie International Edition in
English 1988, 27, 309-332.
70. Zhang, M. Y.; Wesdemiotis, C.; Marchetti, M.; Danis, P. O.; Ray, J. C.; Carpenter, B. K.;
McLafferty, F. W., Characterization of four C4H4 molecules and cations by neutralization-
reionization mass spectrometry. J. Am. Chem. Soc. 1989, 111, 8341-8346.
71. Shay, B. J.; Eberlin, M. N.; Graham Cooks, R.; Wesdemiotis, C., Ion-molecule reactions
and collision-activated dissociation of C4H4
+·
isomers: a case study in the use of the MS3
capabilities of a pentaquadrupole mass spectrometer. J. Am. Soc. Mass. Spectrom. 1992, 3, 518-
534.
72. Kreile, J.; Münzel, N.; Schweig, A.; Specht, H., Uv photoelectron spectrum of
cyclobutadiene. free cyclobutadiene stable up to high temperatures. Chem. Phys. Lett. 1986, 124,
140-146.
73. Kohn, D. W.; Chen, P., Vibrational structure in the photoelectron spectrum of
cyclobutadiene as a probe of structure. J. Am. Chem. Soc. 1993, 115, 2844-2848.
74. Shiotani, M.; Ohta, K.; Nagata, Y.; Sohma, J., Novel cycloaddition of dimethylacetylene
to the dimethylacetylene radical cation: direct observation by ESR. J. Am. Chem. Soc. 1985, 107,
2562-2564.
75. Hrouda, V.; Roeselová, M.; Bally, T., The C4H4
•+
Potential Energy Surface. 3. The
Reaction of Acetylene with Its Radical Cation. J. Phys. Chem. A 1997, 101, 3925-3935.
76. Waite, J. H.; Young, D. T.; Cravens, T. E.; Coates, A. J.; Crary, F. J.; Magee, B.; Westlake,
J., The Process of Tholin Formation in Titan's Upper Atmosphere. Science 2007, 316, 870-875.
77. Hrodmarsson, H. R.; Loison, J. C.; Jacovella, U.; Holland, D. M. P.; Boyé-Péronne, S.;
Gans, B.; Garcia, G. A.; Nahon, L.; Pratt, S. T., Valence-Shell Photoionization of C4H5: The 2-
Butyn-1-yl Radical. J. Phys. Chem. A 2019, 123, 1521-1528.
78. Muller, G.; Jacovella, U.; Catani, K. J.; da Silva, G.; Bieske, E. J., Electronic Spectrum and
Photodissociation Chemistry of the 1-Butyn-3-yl Cation, H3CCHCCH
+
. J. Phys. Chem. A 2020,
124, 2366-2371.
135
79. Lang, M.; Holzmeier, F.; Hemberger, P.; Fischer, I., Threshold Photoelectron Spectra of
Combustion Relevant C4H5 and C4H7 Isomers. J. Phys. Chem. A 2015, 119, 3995-4000.
80. Olah, G. A.; Staral, J. S.; Liang, G., Novel aromatic systems. I. Homocyclopropenyl cation,
the simplest 2.pi. homoaromatic system. J. Am. Chem. Soc. 1974, 96, 6233-6235.
81. Olah, G. A.; Staral, J. S.; Spear, R. J.; Liang, G., Novel aromatic systems. II. Cyclobutenyl
cations and the question of their homoaromaticity. Preparation and study of the
homocyclopropenium ion, the simplest homoaromatic system. J. Am. Chem. Soc. 1975, 97, 5489-
5497.
82. Franke, W.; Schwarz, H.; Stahl, D., Experimental evidence for the existence of stable 1-
cyclobutenyl and cyclopropylidenemethyl cations in the gas phase. The Journal of Organic
Chemistry 1980, 45, 3493-3496.
83. Lossing, F. P.; Holmes, J. L., Stabilization energy and ion size in carbocations in the gas
phase. J. Am. Chem. Soc. 1984, 106, 6917-6920.
84. Douberly, G. E.; Ricks, A. M.; Ticknor, B. W.; McKee, W. C.; Schleyer, P. v. R.; Duncan,
M. A., Infrared Photodissociation Spectroscopy of Protonated Acetylene and Its Clusters. J. Phys.
Chem. A 2008, 112, 1897-1906.
85. Cunje, A.; Lien, M. H.; Hopkinson, A. C., The C4H5
+
Potential Energy Surface. Structure,
Relative Energies, and Enthalpies of Formation of Isomers of C4H5
+
. The Journal of Organic
Chemistry 1996, 61, 5212-5220.
86. Roberts, J. D.; Mazur, R. H., Small-Ring Compounds. IV. Interconversion Reactions of
Cyclobutyl, Cyclopropylcarbinyl and Allylcarbinyl Derivatives. J. Am. Chem. Soc. 1951, 73,
2509-2520.
87. Saunders, M.; Laidig, K. E.; Wiberg, K. B.; Schleyer, P. v. R., Structures, energies, and
modes of interconversion of C4H7
+
ions. J. Am. Chem. Soc. 1988, 110, 7652-7659.
88. Staral, J. S.; Yavari, I.; Roberts, J. D.; Prakash, G. K. S.; Donovan, D. J.; Olah, G. A., Low-
temperature carbon-13 nuclear magnetic resonance spectroscopic investigation of C4H7
+
. Evidence
for an equilibrium involving the nonclassical bicyclobutonium ion and the bisected
cyclopropylcarbinyl cation. J. Am. Chem. Soc. 1978, 100, 8016-8018.
136
89. Myhre, P. C.; Webb, G. G.; Yannoni, C. S., Magic angle spinning nuclear magnetic
resonance near liquid-helium temperatures. Variable-temperature CPMAS studies of C4H7
+
to 5
K. J. Am. Chem. Soc. 1990, 112, 8992-8994.
90. Vančik, H.; Gabelica, V.; Sunko, D. E.; Buzek, P.; v. R. Schleyer, P., Vibrational spectra
of C4H isomers in low-temprature antimony pentafluoride matrices. J. Phys. Org. Chem. 1993, 6,
427-432.
91. Schultz, J. C.; Houle, F. A.; Beauchamp, J. L., Photoelectron Spectroscopy of Isomeric
C4H7 Radicals - Implications for the thermochemistry and Structures of Radicals and their
corresponding Carbonium Ions J. Am. Chem. Soc. 1984, 106, 7336-7347.
92. Koch, W.; Lui, B.; DeFrees, D. J., The C4H7
+
cation. A theoretical investigation. Journal
of American Chemical Society 1988, 110, 7325-7328.
137
Chapter 7. Conclusion and future outlooks
In this work we used electron impact ionization for the first time to generate ions embedded
in helium droplets and record their IR spectra by using action spectroscopy upon laser irradiation.
The low temperature in helium droplets of ~0.4 K ensures that ions are in their ground states and
the obtained spectra are free from the hot bands, which often complicate the spectra at higher
temperatures. We use rather simple experimental set up which can be easily incorporated into the
existing helium droplet spectrometers by adding an external ionizer. Comparison of the spectra of
Zundel cations in He droplets with previous measurements with IRPD show the band width is
about 10 times narrower in helium droplets with almost zero background noise. The superfluid
homogenous environment of the helium droplets provides a weakly interacting "gas phase like"
conditions. Thus, the embedded ions retain their symmetry as it was shown for Zundel cations.
The IR signal obtained in this work is non-linear. This is related to multiple photon absorptions
that are required for the thermal evaporation of droplet and release of the free ions. Henceforth,
the weak bands appear even weaker in the spectra due to this non-linear effect.
We have studied the effect of helium environment on the free rotation of light symmetric
ion-CH3
+
. From the analysis of the rotationally resolved spectrum of v3 C-H antisymmetric stretch
we found that at least two helium atoms bind along the figure axis of the ion changing CH3
+
to
behave as prolate top in helium droplets. This observation is in disagreement with the previously
proposed structure of rigid Atkins snowball around an ion in liquid helium. This is the first
microscopic experimental studies on the structure and dynamics of Atkins snowball around
molecular ions.
138
We have examined the reactivity of hydrocarbons obtained upon electron impact ionization
in helium droplets at ultracold temperatures. Ion-molecule reactions of unsaturated ethylene in
helium droplets have been examined. In addition to the ions from fragmentation of ethylene,
infrared spectra of higher carbon chain ions such as C3H3
+
, C4H5
+
, C4H7
+
etc., resulting from
secondary ion-molecule reactions of ethylene have been obtained. Ion-molecule reactions were
observed with acetylene doped droplets and the infrared spectra of the resulting ions C4H2
+
, C4H3
+
,
C4H4
+
are reported. However, for saturated ethane molecules, it was observed that clustering is
favored. Instead of covalently bound species, weaker bound dimers such as (C2H6)(C2H4)
+
,
(C2H6)(C2H5)
+
, and (C2H6)(C2H6)
+
, as well as larger clusters containing several ethane molecules
attached to C2H4
+
, C2H5
+
, and C2H6
+
ionic cores were observed. The IR spectra obtained for the
carbocations are highly resolved with widths of the order of 1-3 cm
-1
, and matrix shifts of less than
20 cm
-1
are obtained from their respective gas phase values, when available. In addition, IR spectra
of a number of carbocations such as C3H6
+
, C4H2
+
, C4H3
+
, C4H5
+
, C4H7
+
have been reported for
the first time. Further work and quantum calculations are needed to understand the structure of
these ions.
We also studied the different isomers of various carbocations resulting from different
precursors such as n-butane, transbutene, propene, allene, acetylene, ethylene. Due to the ultracold
environment of the helium droplet instant freezing of the ions occurs which results in capture of
the local minima of the structure not just the global minima. Henceforth, with helium droplets we
have access to the different isomer structures of carbocation depending on the initial precursor.
Linear structures of carbocations such as C3H3
+
,
C3H2
+
etc., where obtained to be the dominant
isomers when linear precursor molecules were used. On the other hands the thermodynamically
stable structures are often cyclic. In the future cyclic precursors such as cyclopropane or
139
cyclobutene should be used to further understand the dependence of the isomers generated and
their composition on the precursor used.
This thesis covered the vibrational ion spectroscopy in helium droplets - the
instrumentation, quality of the IR spectra obtained and various aspects such as reactivity, isomer
dependence on precursor, interaction with He environment, classical/non-classical structure of
different ions obtained. However, a lot remains to be explored and understood in this relatively
new method – both improvements needed in the instrumental set up to acquire more scientific data
and the systems/the scientific problems that can be studied with it.
In the current experimental setup, (Fig. 2.5) upon ionization in external ionizer the cationic
droplets traverse a distance of ~ 15 cm towards the ionization region of the probe ionizer of QMS,
whereas light ions are rejected by an Einzel lens stack acting as a high pass filter. The intensity
and quality of signal obtained is highly dependent on the voltages on this Einzel lens and the ion
region orifice. Replacing the existing Einzel lens stack with an octupole RF ion guide will function
more effectively as a high pass filter. Installation of the ion guide is also expected to increase the
signal due to more complete transport of smaller cationic droplets to the laser interaction range
and elimination of the stray field effect on the signal. The IR signal obtained in the current method
requires multiphoton excitation, which complicates the identification of weak IR bands. This
problem could be solved by using smaller droplets which could be produced by starting with
production of ions in larger droplets followed by subsequent moderation of the droplet size by
multiple collisions with He atoms in RF ion guide. The laser used for these experiments has
optimum performance in the range of 2600-4000 cm
-1
, the operational range could be extended
down to 600 cm
-1
by adding another stage of difference frequency generation with AgGaSe 2
crystal. This extension will allow to obtain IR spectrum of the "fingerprint" vibrations.
140
We studied number of ions resulting from fragmentation, ion-molecule reactions and
clustering of the ionized doped droplets. However, found that attaining the spectra of the
protonated hydrocarbon cations like CH5
+
, C2H5
+
, C2H7
+
is hard to achieve. Controlled
experiments by adding hydrogen in detection chamber or RF guide to the ionized droplets can be
tried in future to study these protonated species. In addition, the carbocations obtained via ion-
molecule reactions in this work used only one precursor, ion-molecule reactions from two different
reactants can be studied by doping the droplets with two different precursors, such as ethane and
acetylene. We have studied water ion and Zundel cation. These experiments could be extended to
study larger water clusters (trimers, tetramers) which will enable understanding the solvation
mechanism better. This technique could straightforwardly be extended for preparation and study
of diverse C-, N-, and O-based cations of different sizes.
Abstract (if available)
Abstract
Carbocations play an important role as reactive intermediates in both laboratory and astrophysical environments. Understanding their structure and stability allows one to better estimate favorable reaction pathways and design chemical reactions. In addition to NMR, vibrational infrared (IR) spectroscopy can also provide information on the structure of ions. However, in comparison to neutral molecular species, IR spectroscopy of ions is much less developed primarily to their high reactivity. This thesis presents the experiments on the IR spectroscopy of carbocations in superfluid helium droplets which employ electron impact (EI) ionization for the preparation of the ions. Due to the low temperature (~0.4 K) and homogeneous superfluid environment of the embedded species the droplets present an ultimate matrix for spectroscopic measurements. Because the ionization potential of He of ~24 eV is way above that for any other species, any molecular ions can be produced and studied in the droplets. Here we show that the EI of the droplets containing neutral precursor molecules yields ionic species embedded in the droplets of few thousands of atoms, making it a convenient nanomatrix for production and spectroscopic interrogation of a wide variety of embedded ions and ionic clusters. Infrared spectra are obtained using release of the cations from the droplets upon laser excitation. The topics of interest in this thesis include - cation-He interaction, ion – molecule and clustering reactions at ultracold temperatures. Rotational resolved spectrum of CH3+ ions was used to study the interaction of a light rotor ion with helium environment. Previously it was assumed that He around ions form a rigid "snowball" structure. We concluded that the effective rotor in helium has two He atoms attached on the figure axis of CH3+ with other He atoms in the first shell remain fluxional. Besides, we report IR spectra of ions such as C2H2+, C2H3+, C2H4+, C3H3+, C4H5+,C4H7+ etc., resulting from ionization, fragmentation and ion-molecule reactions of unsaturated ethylene doped helium droplets upon electron impact ionization. Interestingly for saturated ethane clustering was found to be favored over ion-molecule reactions at the low temperature in helium droplet. In addition, we extend our experiments to various isomers of ions resulting from electron impact ionization of droplets doped with various precursor molecules such as acetylene, transbutene and propene. Many of the obtained IR spectra such as for C3H6+, C4H3+, C4H4+, C4H5+, C4H7+, C2H+ are reported for the first time in this thesis. The spectra will help understanding the structure of these reactive carbocations.
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Asset Metadata
Creator
Erukala, Swetha
(author)
Core Title
Infrared spectroscopy of carbocations in helium droplets
School
College of Letters, Arts and Sciences
Degree
Doctor of Philosophy
Degree Program
Chemistry
Degree Conferral Date
2022-08
Publication Date
07/09/2023
Defense Date
05/11/2022
Publisher
University of Southern California
(original),
University of Southern California. Libraries
(digital)
Tag
carbocations,electron impact ionization,helium droplets,infrared spectroscopy,ions,non-classical carbocations,OAI-PMH Harvest,Zundel
Format
application/pdf
(imt)
Language
English
Contributor
Electronically uploaded by the author
(provenance)
Advisor
Vilesov, Andrey (
committee chair
), Kresin, Vitaly (
committee member
), Prezhdo, Oleg (
committee member
)
Creator Email
erukala@usc.edu,shwetha1120@gmail.com
Permanent Link (DOI)
https://doi.org/10.25549/usctheses-oUC111371073
Unique identifier
UC111371073
Legacy Identifier
etd-ErukalaSwe-10825
Document Type
Dissertation
Format
application/pdf (imt)
Rights
Erukala, Swetha
Type
texts
Source
20220713-usctheses-batch-952
(batch),
University of Southern California
(contributing entity),
University of Southern California Dissertations and Theses
(collection)
Access Conditions
The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the author, as the original true and official version of the work, but does not grant the reader permission to use the work if the desired use is covered by copyright. It is the author, as rights holder, who must provide use permission if such use is covered by copyright. The original signature page accompanying the original submission of the work to the USC Libraries is retained by the USC Libraries and a copy of it may be obtained by authorized requesters contacting the repository e-mail address given.
Repository Name
University of Southern California Digital Library
Repository Location
USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA
Repository Email
cisadmin@lib.usc.edu
Tags
carbocations
electron impact ionization
helium droplets
infrared spectroscopy
ions
non-classical carbocations
Zundel