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qualitatively similar to those obtained from liquid expansion of the neat p-H2/D2 liquid.
The frequency of the Q1(0) line rises linearly with Y, but has somewhat smaller slope of
1.6 cm-1 as compared with 2.7 cm-1 at X = 100%. The discrepancy between observed
slope of the frequency dependence at X = 8% and 100% is not completely understood.
Comparison of panels (a) and (b) in Fig. 5.3 shows that neat p-H2 clusters obtained at X =
8% have about 0.6 cm-1 higher frequency as compared with clusters obtained at X =
100%. This may indicate that clusters obtained at X = 8% have lower density than in the
bulk, which will also result in lower rate of vibron hopping than in bulk hydrogen. It
should be noted, that the frequency shift is not due to temperature dependence, as it is
very weak. Previous high resolution measurements show that the frequency of the Q1(0)
line in bulk crystalline solid rises approximately 0.13 cm-1 from 4.9 to 13.5 K.21 Thus
we conclude that clusters obtained at X = 100% and 8% may have somewhat different
structure and density, such as polycrystalline and amorphous. Different structures of the
clusters may also influence the width of the vibrational band, which is expected to be
smaller in less regular clusters. Finally, the frequency may be influenced by some phase
separation, as it will be discussed in more details in the case of X = 1%.
We proceed with discussion of the results obtained at X = 1%. Fig. 5.3 (c) shows
that for small L and increasing D2 content, the frequency of the Q1(0) line first rises by
about 0.4 cm-1 up to Y = 35%. At larger Y, the frequency of the line remains
approximately constant at 4151.52 ± 0.02 cm-1. At large L, the frequency is about
4151.29 ± 0.05 cm-1, only slightly higher than in neat pH2 clusters at 4151.01 ± 0.05 cm-1.
The saturation of the frequency dependence at Y ≥ 35%, in terms of vibron hopping,
126
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 150 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | qualitatively similar to those obtained from liquid expansion of the neat p-H2/D2 liquid. The frequency of the Q1(0) line rises linearly with Y, but has somewhat smaller slope of 1.6 cm-1 as compared with 2.7 cm-1 at X = 100%. The discrepancy between observed slope of the frequency dependence at X = 8% and 100% is not completely understood. Comparison of panels (a) and (b) in Fig. 5.3 shows that neat p-H2 clusters obtained at X = 8% have about 0.6 cm-1 higher frequency as compared with clusters obtained at X = 100%. This may indicate that clusters obtained at X = 8% have lower density than in the bulk, which will also result in lower rate of vibron hopping than in bulk hydrogen. It should be noted, that the frequency shift is not due to temperature dependence, as it is very weak. Previous high resolution measurements show that the frequency of the Q1(0) line in bulk crystalline solid rises approximately 0.13 cm-1 from 4.9 to 13.5 K.21 Thus we conclude that clusters obtained at X = 100% and 8% may have somewhat different structure and density, such as polycrystalline and amorphous. Different structures of the clusters may also influence the width of the vibrational band, which is expected to be smaller in less regular clusters. Finally, the frequency may be influenced by some phase separation, as it will be discussed in more details in the case of X = 1%. We proceed with discussion of the results obtained at X = 1%. Fig. 5.3 (c) shows that for small L and increasing D2 content, the frequency of the Q1(0) line first rises by about 0.4 cm-1 up to Y = 35%. At larger Y, the frequency of the line remains approximately constant at 4151.52 ± 0.02 cm-1. At large L, the frequency is about 4151.29 ± 0.05 cm-1, only slightly higher than in neat pH2 clusters at 4151.01 ± 0.05 cm-1. The saturation of the frequency dependence at Y ≥ 35%, in terms of vibron hopping, 126 |
