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5.4. Discussion
It is important to review previous results regarding the physical origin of the
vibrational frequency shift in condensed phase p-H2 and its mixtures.15,16,21,23,24 The
frequency of the Q1(0) line in solid is about 11.4 cm-1 lower than that observed in the gas
phase20 at 4161.1687 cm-1. The lowering of the frequency is mainly accounted for by an
8.7 cm-1 shift due to isotropic dispersion intermolecular interactions.24 In addition, the
vibron excitation is easily delocalized between p-H2 molecules in the solid giving rise to
a vibrational band which has width of about 3.6 cm-1.15,22 However, due to momentum
conservation, which dictates the selections rules in the Raman spectrum15, only
transitions to the lowest level of the vibron band are allowed, thereby producing a very
narrow Q1(0) line. As a result, the frequency of the Q1(0) line has an additional 2.7 cm-1
downward shift.24
Formation of the vibron band in liquid has been studied to a much lesser degree.
Measurements of the intensity ratio of the Q1(0) and Q1(1) lines in liquid H2 show the
same intensity enhancement of the Q1(1) line as in solid indicating similar vibron band
formation.25 Our recent study of the temperature dependence of the frequency of the
Q1(0) line in pure pH2 shows that the frequency change of the Q1(0) line in liquid and in
solid can be accounted for by the same function of density, indicating similar origin of the
line shift in solid and in liquid. As a result, we assume that the magnitude of the vibron
shift of the Q1(0) line in liquid is approximately the same as in solid. If we assume that
the width of the vibron band scales with the total shift of the Q1(0) line from the gas
phase, about 10% smaller band width is expected in low temperature liquid than in solid
123
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 147 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 5.4. Discussion It is important to review previous results regarding the physical origin of the vibrational frequency shift in condensed phase p-H2 and its mixtures.15,16,21,23,24 The frequency of the Q1(0) line in solid is about 11.4 cm-1 lower than that observed in the gas phase20 at 4161.1687 cm-1. The lowering of the frequency is mainly accounted for by an 8.7 cm-1 shift due to isotropic dispersion intermolecular interactions.24 In addition, the vibron excitation is easily delocalized between p-H2 molecules in the solid giving rise to a vibrational band which has width of about 3.6 cm-1.15,22 However, due to momentum conservation, which dictates the selections rules in the Raman spectrum15, only transitions to the lowest level of the vibron band are allowed, thereby producing a very narrow Q1(0) line. As a result, the frequency of the Q1(0) line has an additional 2.7 cm-1 downward shift.24 Formation of the vibron band in liquid has been studied to a much lesser degree. Measurements of the intensity ratio of the Q1(0) and Q1(1) lines in liquid H2 show the same intensity enhancement of the Q1(1) line as in solid indicating similar vibron band formation.25 Our recent study of the temperature dependence of the frequency of the Q1(0) line in pure pH2 shows that the frequency change of the Q1(0) line in liquid and in solid can be accounted for by the same function of density, indicating similar origin of the line shift in solid and in liquid. As a result, we assume that the magnitude of the vibron shift of the Q1(0) line in liquid is approximately the same as in solid. If we assume that the width of the vibron band scales with the total shift of the Q1(0) line from the gas phase, about 10% smaller band width is expected in low temperature liquid than in solid 123 |
