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greater than or equal to the conversion rate of single water molecules. Thus, we conclude
that the splitting around 3736 cm-1 is a site effect due to the Ar matrix and that spin
relaxation in the dimer is fast in the scale of a few minutes.
A recent work has focused on water dimers in a variety of matrices.40 For solid
Ar, an indication of acceptor switching was observed in the ν3 region based on the
presence of split features at 3736.0 and 3737.7 cm-1, similar to what was observed in the
current work at 3735.4 and 3737.5 cm-1. In addition, both these features are believed to
originate from the ground state implying para-H2O dimers. However due to selection
rules, as dictated by the nuclear spin wavefunctions, transitions originating from para-
(H2O)2 to anti-symmetric ro-vibrational states are forbidden. As a result at low
temperature, if dimers have relaxed, only a single feature should occur as is observed in
the current work. On the other hand, we have occasionally observed a second feature
located around 3725 cm-1 which was attributed to dimer complexes with N2 as observed
previously.21 However, the intensity of the feature decreased irreversibly over time
indicating a conversion but without the subsequent increase of para features. With
constant annealing at T = 30 K and subsequent re-cooling to T = 4 K, the irregular feature
was not restored despite the onset of intensity from single molecule ortho lines. Upon
stringent purification methods of the experimental setup, the feature was absent from
later spectra indicating a possible impurity effect.
Infrared spectra of water molecules in cometary tails have indicated low OPR of
about 2.5:1, which is consistent with a spin temperature of about 30 K.4,5 As a result, it is
generally believed that the obtained OPR could be a measure of the temperature at which
177
Object Description
| Title | Infrared and Raman spectrosopy of molecules and molecular aggregates in helium droplets |
| Author | Sliter, Russell Thomas |
| Author email | sliter@usc.edu; sliterr@gmail.com |
| Degree | Doctor of Philosophy |
| Document type | Dissertation |
| Degree program | Chemistry |
| School | College of Letters, Arts and Sciences |
| Date defended/completed | 2011-04-21 |
| Date submitted | 2011 |
| Restricted until | Unrestricted |
| Date published | 2011-04-26 |
| Advisor (committee chair) | Vilesov, Andrey F. |
| Advisor (committee member) |
Reisler, Hannah Kresin, Vitaly V. |
| Abstract | This dissertation covers several different aspects of spectroscopy of molecules and molecular clusters embedded in low-temperature matrices, such as helium droplets. First, details on the formation and optimization of He droplets will be discussed. A new method of measuring droplet sizes for cw nozzle expansions using mass spectrometry was developed. The results of the measurements of the sizes of the droplets in pulsed expansion as a function of temperature will be described. Details on the electron-impact ionization of He droplets will also be discussed as well as a simple method of modeling the ionization and excitation of He atoms in the droplet. In addition, preliminary measurements on the size distribution of He droplets produced at very low temperature of 5 – 7 K in continuous expansion will be addressed.; Using matrix isolation in He droplets, vibrational spectra of clusters consisting of para-H₂ or para-H₂/D₂ have been obtained using coherent anti-Stokes Raman spectroscopy (CARS). The vibrational frequency of para-H₂ molecules obtained upon expansion of neat para-H₂/D₂ gas or liquid was found to be very similar to that in bulk solid samples having equal composition. On the other hand, spectra in clusters obtained upon expansion of 1% para-H₂/D₂ clusters seeded in He are liquid and have a considerable frequency shift, which indicate phase separation of the two isotopes in clusters at low temperature. The onset of phase separation in para-H₂/D₂ mixtures is predicted at approximately 3 K providing further evidence of super-cooled liquid hydrogen clusters.; To address the Raman spectra observed in liquid H2 clusters, vibrational and rotational spectra of bulk liquid para-H2 at temperature of T = 14 – 26 K and of solid at T = 6 – 13 K have been obtained using coherent anti-Stokes Raman scattering technique. The vibrational frequency in the liquid increases with temperature by about 2 cm⁻¹, and the shift scales with the square of the sample’s density. An extrapolation of the vibrational frequency in the metastable para-H₂ liquid below the freezing point is discussed. The results indicate that the vibron hopping between the molecules is active in the liquid, similar to that previously found in the solid.; Matrix isolation has also been performed in argon solid matrices based on a custom-made cryogenic optical cell. Single water molecules have been isolated in solid Ar matrices at 4 K and studied by ro-vibrational spectroscopy using FTIR in the regions of the v₁, v₂, and v₃ modes. Upon nuclear spin conversion at 4 K, essentially pure para-H₂O was prepared followed by subsequent fast annealing generating ice particles. FTIR studies of the vapor above the condensed water upon annealing to T ≥ 250 K indicate fast re-conversion of nuclear spin to equilibrium conditions. Our results indicate that nuclear spin conversion is fast in water dimers and larger clusters, which preclude preparation of concentrated samples of para-H₂O, such as in ice or vapor. |
| Keyword | Helium droplets; laser spectroscopy; matrix isolation; superfluidity; clusters |
| Language | English |
| Part of collection | University of Southern California dissertations and theses |
| Publisher (of the original version) | University of Southern California |
| Place of publication (of the original version) | Los Angeles, California |
| Publisher (of the digital version) | University of Southern California. Libraries |
| Provenance | Electronically uploaded by the author |
| Type | texts |
| Legacy record ID | usctheses-m3778 |
| Contributing entity | University of Southern California |
| Rights | Sliter, Russell Thomas |
| Repository name | Libraries, University of Southern California |
| Repository address | Los Angeles, California |
| Repository email | cisadmin@lib.usc.edu |
| Filename | etd-Sliter-4404 |
| Archival file | uscthesesreloadpub_Volume23/etd-Sliter-4404.pdf |
Description
| Title | Page 201 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | greater than or equal to the conversion rate of single water molecules. Thus, we conclude that the splitting around 3736 cm-1 is a site effect due to the Ar matrix and that spin relaxation in the dimer is fast in the scale of a few minutes. A recent work has focused on water dimers in a variety of matrices.40 For solid Ar, an indication of acceptor switching was observed in the ν3 region based on the presence of split features at 3736.0 and 3737.7 cm-1, similar to what was observed in the current work at 3735.4 and 3737.5 cm-1. In addition, both these features are believed to originate from the ground state implying para-H2O dimers. However due to selection rules, as dictated by the nuclear spin wavefunctions, transitions originating from para- (H2O)2 to anti-symmetric ro-vibrational states are forbidden. As a result at low temperature, if dimers have relaxed, only a single feature should occur as is observed in the current work. On the other hand, we have occasionally observed a second feature located around 3725 cm-1 which was attributed to dimer complexes with N2 as observed previously.21 However, the intensity of the feature decreased irreversibly over time indicating a conversion but without the subsequent increase of para features. With constant annealing at T = 30 K and subsequent re-cooling to T = 4 K, the irregular feature was not restored despite the onset of intensity from single molecule ortho lines. Upon stringent purification methods of the experimental setup, the feature was absent from later spectra indicating a possible impurity effect. Infrared spectra of water molecules in cometary tails have indicated low OPR of about 2.5:1, which is consistent with a spin temperature of about 30 K.4,5 As a result, it is generally believed that the obtained OPR could be a measure of the temperature at which 177 |
