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lifetime of p-H2O molecules in Ar matrices at higher temperatures.9 Very recently,
however, enrichment of ortho- and para- water via gas phase chromatography has been
reported with surprisingly long conversion times in liquid of about 30 min.8,14
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 of the
primordial solar system.15 Conversely, some recent works have questioned the
enhancement of para-H2O due to its negligible vapor pressure at 30 K and the extent of
which the temperature of the water vapor corresponds to the temperature of the comet
ice.16
In this work, we studied the feasibility of the formation of p-H2O ices. We began
with the nuclear spin conversion of very diluted samples of water in Ar matrices that have
been spin converted at T = 4 K in an enclosed cryogenic optical cell. The ice particles are
formed from p-H2O molecules upon fast increase of the temperature and removal of the
Ar constituency from the cell. Finally, infrared spectra of the gas in the cell have been
obtained at T = 260 – 280 K. The acquired spectra are identical with those of
unconverted water, showing that the lifetime of p-H2O in ice is less than about 30 min. In
addition, our results indicate that spin conversion takes place already in water dimers.
157
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 181 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | lifetime of p-H2O molecules in Ar matrices at higher temperatures.9 Very recently, however, enrichment of ortho- and para- water via gas phase chromatography has been reported with surprisingly long conversion times in liquid of about 30 min.8,14 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 of the primordial solar system.15 Conversely, some recent works have questioned the enhancement of para-H2O due to its negligible vapor pressure at 30 K and the extent of which the temperature of the water vapor corresponds to the temperature of the comet ice.16 In this work, we studied the feasibility of the formation of p-H2O ices. We began with the nuclear spin conversion of very diluted samples of water in Ar matrices that have been spin converted at T = 4 K in an enclosed cryogenic optical cell. The ice particles are formed from p-H2O molecules upon fast increase of the temperature and removal of the Ar constituency from the cell. Finally, infrared spectra of the gas in the cell have been obtained at T = 260 – 280 K. The acquired spectra are identical with those of unconverted water, showing that the lifetime of p-H2O in ice is less than about 30 min. In addition, our results indicate that spin conversion takes place already in water dimers. 157 |
