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One of the unique properties of liquid He is the phase separation of 3He/4He
mixtures at temperature below 0.87 K.11 At very low temperatures, the mixture of 3He
and 4He consists of nearly pure 3He liquid on the surface and a mixture of 8% 3He in a
4He core. Phase separation has also been predicted in mixtures of p-H2 and D2 at
temperatures ≤ 3 K12,13, where it derives from different zero point energies in neat and
mixed substances. At these low temperatures, hydrogen is solid and thus no phase
separation has been directly detected due to very slow diffusion rate in solid.14 As a
result, the goal of this work is to observe phase separation in cold liquid p-H2/D2 clusters
via vibrational CARS spectroscopy of p-H2 molecules in the cluster.
In H2 condensed phases, vibrations form a band having width of about 3 cm-1.15
Dilution of p-H2 with D2 molecules leads to a decrease of the efficiency of the vibron
hopping and thus smaller band width. Therefore, the measured vibrational frequency of
the p-H2 molecules is a direct probe of the fraction of the D2 molecules in the p-H2
liquid.16 We have obtained that the mole fraction of D2 in solid clusters is very close to
that in the expanding gas mixture. On the other hand, D2 content in liquid clusters is
depleted by up to a factor of five with respect to that in the expanding gas. These results
indicate phase separation into nearly pure p-H2 and D2 phases in the surface and interior
of the cluster, respectively.
115
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 139 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | One of the unique properties of liquid He is the phase separation of 3He/4He mixtures at temperature below 0.87 K.11 At very low temperatures, the mixture of 3He and 4He consists of nearly pure 3He liquid on the surface and a mixture of 8% 3He in a 4He core. Phase separation has also been predicted in mixtures of p-H2 and D2 at temperatures ≤ 3 K12,13, where it derives from different zero point energies in neat and mixed substances. At these low temperatures, hydrogen is solid and thus no phase separation has been directly detected due to very slow diffusion rate in solid.14 As a result, the goal of this work is to observe phase separation in cold liquid p-H2/D2 clusters via vibrational CARS spectroscopy of p-H2 molecules in the cluster. In H2 condensed phases, vibrations form a band having width of about 3 cm-1.15 Dilution of p-H2 with D2 molecules leads to a decrease of the efficiency of the vibron hopping and thus smaller band width. Therefore, the measured vibrational frequency of the p-H2 molecules is a direct probe of the fraction of the D2 molecules in the p-H2 liquid.16 We have obtained that the mole fraction of D2 in solid clusters is very close to that in the expanding gas mixture. On the other hand, D2 content in liquid clusters is depleted by up to a factor of five with respect to that in the expanding gas. These results indicate phase separation into nearly pure p-H2 and D2 phases in the surface and interior of the cluster, respectively. 115 |
