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xxi
FIG. 7.1. 158
Schematic of cryogenic copper cell. The 3-cm long copper cell has an 18 mm diameter
optical clearance enclosed by two 3-mm thick CaF2 windows. The adjacent side of the
cell provides a 9 mm clearance in which a stainless steel adapter with 1/16” thick copper
tubing is attached for gas mixture introduction.
FIG. 7.2. 160
Energy level diagram of water molecules. Rotational level labels to the left of the lines
represent JKaKc and correspond with the ortho- and para- labels on the right; o and p,
respectively. Solid arrows indicate observed ro-vibrational transitions in ν1, ν2, and ν3
bands at low temperature.
FIG. 7.3. 161
IR absorption spectra of the ν1, ν2, and ν3 regions of H2O of a 1:2000 H2O:Ar sample.
Panels (a) and (b) refer to spectra obtained after completing ~ 70 min deposition at a
nominal temperature of 4 K and after an additional 1400 min, respectively.
FIG. 7.4. 164
IR absorption spectra of the ν1, ν2 and ν3 regions of H2O in a 1:2000 H2O:Ar sample at 4
K (a) and 50 K (b). Timeline refers to spectra obtained after completing ~ 70 min
deposition of mixture.
FIG. 7.5. 167
IR spectra in the ν3/ν1 region upon completion of ~ 40 min. 1:1000 H2O:Ar sample
deposition at 4 K (a), ~1730 min. conversion at 4 K (b), annealing to T = 30 K within 1
min (c), after constant heating for 5 min at 30 K (d), and after re-cooling the sample back
to 4 K (e).
FIG. 7.6. 168
IR spectra of a 1:1000 H2O:Ar sample in the ν3/ν1 region upon completion of ~ 40 min.
deposition at 4 K (a), annealing to T = 30 K within 1 min (b), after constant heating for 5
min at 30 K (c), and after re-cooling the sample back to 4 K (d).
FIG. 7.7. 169
Normalized IR absorption spectra of ν1/ν3 region of H2O at various temperatures after
conversion to p-H2O and subsequent fast annealing, (a) – (c). Panel (d) shows
comparable spectra of normal H2O at 1 mbar and T = 295 K.
FIG. 7.8. 170
Magnified spectra of traces (b) - (d) from Fig. 7.7 in the range of 3750 – 3850 cm-1.
Solid lines represent o-H2O transitions while dashed lines represent p-H2O transitions.
Unlabeled lines represent overlaps of ortho and para transitions.
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 21 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | xxi FIG. 7.1. 158 Schematic of cryogenic copper cell. The 3-cm long copper cell has an 18 mm diameter optical clearance enclosed by two 3-mm thick CaF2 windows. The adjacent side of the cell provides a 9 mm clearance in which a stainless steel adapter with 1/16” thick copper tubing is attached for gas mixture introduction. FIG. 7.2. 160 Energy level diagram of water molecules. Rotational level labels to the left of the lines represent JKaKc and correspond with the ortho- and para- labels on the right; o and p, respectively. Solid arrows indicate observed ro-vibrational transitions in ν1, ν2, and ν3 bands at low temperature. FIG. 7.3. 161 IR absorption spectra of the ν1, ν2, and ν3 regions of H2O of a 1:2000 H2O:Ar sample. Panels (a) and (b) refer to spectra obtained after completing ~ 70 min deposition at a nominal temperature of 4 K and after an additional 1400 min, respectively. FIG. 7.4. 164 IR absorption spectra of the ν1, ν2 and ν3 regions of H2O in a 1:2000 H2O:Ar sample at 4 K (a) and 50 K (b). Timeline refers to spectra obtained after completing ~ 70 min deposition of mixture. FIG. 7.5. 167 IR spectra in the ν3/ν1 region upon completion of ~ 40 min. 1:1000 H2O:Ar sample deposition at 4 K (a), ~1730 min. conversion at 4 K (b), annealing to T = 30 K within 1 min (c), after constant heating for 5 min at 30 K (d), and after re-cooling the sample back to 4 K (e). FIG. 7.6. 168 IR spectra of a 1:1000 H2O:Ar sample in the ν3/ν1 region upon completion of ~ 40 min. deposition at 4 K (a), annealing to T = 30 K within 1 min (b), after constant heating for 5 min at 30 K (c), and after re-cooling the sample back to 4 K (d). FIG. 7.7. 169 Normalized IR absorption spectra of ν1/ν3 region of H2O at various temperatures after conversion to p-H2O and subsequent fast annealing, (a) – (c). Panel (d) shows comparable spectra of normal H2O at 1 mbar and T = 295 K. FIG. 7.8. 170 Magnified spectra of traces (b) - (d) from Fig. 7.7 in the range of 3750 – 3850 cm-1. Solid lines represent o-H2O transitions while dashed lines represent p-H2O transitions. Unlabeled lines represent overlaps of ortho and para transitions. |
