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in the bulk. The precise effect of the cluster size on the frequency of the Q1(0) line
remains to be understood. As a result, a more detailed theoretical work for determining
the density below the triple point as well as the role of the size effects are required in
order to describe their effect on the Raman spectrum of the clusters.
The fact that Eq. (6.1) provides a universal fit of the frequency in both liquid and
solid is somewhat surprising. Apart from the larger thermal expansion coefficient, liquid
differs from the solid in at least two other important respects: i) it lacks a long range
order of the molecules, and ii) molecules in the liquid are in a state of Brownian motion,
which causes faster de-phasing of the excitations than in solid. Nevertheless, relatively
narrow widths of the Q1(0) lines in the liquid of about 0.1 cm-1 show that momentum of
the vibrons remains a relatively good quantum number. The results also suggest that
larger delocalization of the H2 molecules in the liquid has a small effect on the frequency.
Thus, our results indicate that the vibron hopping observed previously in the solid is also
effective in the liquid. This is in agreement with previous conclusions based on the
intensity ratio of the Q1(0) and Q1(1) lines in liquids consisting of mixed pH2 and oH2.29
6.5. Conclusions
We have prepared bulk pH2 in both solid and liquid phases and studied their
vibrational and rotational Raman spectra at different temperatures. The vibrational
frequency in the liquid increases with temperature by about 2 cm-1 from 14 to 26 K. The
vibrational frequency was found to scale with the square of the density in the liquid.
These results were used to extrapolate the vibrational frequency in the temperature range
151
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 175 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | in the bulk. The precise effect of the cluster size on the frequency of the Q1(0) line remains to be understood. As a result, a more detailed theoretical work for determining the density below the triple point as well as the role of the size effects are required in order to describe their effect on the Raman spectrum of the clusters. The fact that Eq. (6.1) provides a universal fit of the frequency in both liquid and solid is somewhat surprising. Apart from the larger thermal expansion coefficient, liquid differs from the solid in at least two other important respects: i) it lacks a long range order of the molecules, and ii) molecules in the liquid are in a state of Brownian motion, which causes faster de-phasing of the excitations than in solid. Nevertheless, relatively narrow widths of the Q1(0) lines in the liquid of about 0.1 cm-1 show that momentum of the vibrons remains a relatively good quantum number. The results also suggest that larger delocalization of the H2 molecules in the liquid has a small effect on the frequency. Thus, our results indicate that the vibron hopping observed previously in the solid is also effective in the liquid. This is in agreement with previous conclusions based on the intensity ratio of the Q1(0) and Q1(1) lines in liquids consisting of mixed pH2 and oH2.29 6.5. Conclusions We have prepared bulk pH2 in both solid and liquid phases and studied their vibrational and rotational Raman spectra at different temperatures. The vibrational frequency in the liquid increases with temperature by about 2 cm-1 from 14 to 26 K. The vibrational frequency was found to scale with the square of the density in the liquid. These results were used to extrapolate the vibrational frequency in the temperature range 151 |
