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5.2. Experimental Technique
Hydrogen (p-H2 or mixed p-H2/D2) clusters are formed by a pulsed free jet
expansion of either a liquid or cold gas mixture of hydrogen in He at T0 = 15 – 22 K and
a stagnation pressure of P0 = 3 and 20 bar, respectively. p-H2 is produced by catalytic
conversion of liquid n-H2 (Praxair, research grade) with a residual o-H2 content of less
than 0.1%.17 Normal D2 was used as received. p-H2/D2/He mixtures were prepared in a
1 gal. stainless steel sampling cylinder. Mixtures were generally prepared the day prior
to experiment to ensure sufficient mixing. The combined molar fraction of hydrogen and
deuterium in the He mixture will be referred to as X. X = 100% indicates expansion of
pure hydrogen/deuterium mixtures in the absence of helium. In this work we have
studied clusters which have been prepared from mixtures having X = 1%, 8%, and 100%.
For each X, the molar fraction of D2 in the H2/D2 gas is varied in the range of Y = 0 –
98%.
The cryogenic molecular beam apparatus is described in detail elsewhere.18 The
pulsed valve assembly, used to form the jet expansion, consists of a Series 99 (General
Dynamics Valves Inc.) valve equipped with copper sealing gaskets. The nozzle has a 1
mm diameter with a custom machined conical opening of about 90º. The entire valve
assembly is mounted onto a copper platform which is attached to the second stage of a
closed-cycle helium refrigerator. The pulsed nozzle assembly can be moved in the
horizontal plane. Thus, Raman spectra can be obtained at different distances from the
nozzle – L up to about 50 mm.
116
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 140 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 5.2. Experimental Technique Hydrogen (p-H2 or mixed p-H2/D2) clusters are formed by a pulsed free jet expansion of either a liquid or cold gas mixture of hydrogen in He at T0 = 15 – 22 K and a stagnation pressure of P0 = 3 and 20 bar, respectively. p-H2 is produced by catalytic conversion of liquid n-H2 (Praxair, research grade) with a residual o-H2 content of less than 0.1%.17 Normal D2 was used as received. p-H2/D2/He mixtures were prepared in a 1 gal. stainless steel sampling cylinder. Mixtures were generally prepared the day prior to experiment to ensure sufficient mixing. The combined molar fraction of hydrogen and deuterium in the He mixture will be referred to as X. X = 100% indicates expansion of pure hydrogen/deuterium mixtures in the absence of helium. In this work we have studied clusters which have been prepared from mixtures having X = 1%, 8%, and 100%. For each X, the molar fraction of D2 in the H2/D2 gas is varied in the range of Y = 0 – 98%. The cryogenic molecular beam apparatus is described in detail elsewhere.18 The pulsed valve assembly, used to form the jet expansion, consists of a Series 99 (General Dynamics Valves Inc.) valve equipped with copper sealing gaskets. The nozzle has a 1 mm diameter with a custom machined conical opening of about 90º. The entire valve assembly is mounted onto a copper platform which is attached to the second stage of a closed-cycle helium refrigerator. The pulsed nozzle assembly can be moved in the horizontal plane. Thus, Raman spectra can be obtained at different distances from the nozzle – L up to about 50 mm. 116 |
