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4.4. H2 Cluster Formation via Supersonic Co-expansion of H2 and He.
In general, He droplets formed by supersonic expansion are doped with atomic or
molecular species in a pickup chamber. As it follows from eq. (4.2), CARS signal
depends as a square of the number density signifying a 4th power dependence on the
CARS signal with increasing distance from the nozzle. Therefore, besides expansion of a
liquid jet of neat H2, experiments can only be performed at distances no more than 5 cm
away from the nozzle. Using a pickup cell is not practical, as it must be accommodated
in the high density range of the expansion and thus will interfere with the expansion. As
a result, for CARS experiments, we perform either a supersonic co-expansion of H2
seeded in He or an expansion of neat H2.
4.5. Sample Preparation
Parahydrogen was mixed with He in a 1 gal. stainless steel sampling cylinder. p-
H2 having o-H2 content of less than 0.01% was prepared by passing liquid n-H2 (research
grade) over a paramagnetic material at temperature of about 16 K.5 Sample mixtures
were prepared by first purging the sample cylinder with helium gas. Afterward, known
amounts of p-H2 were added and subsequently filled up with He gas to pressures no
higher than 40 bar. Total pressures of 40 bar were necessary to perform experiments for
multiple days, typically never exceeding 3 days use to ensure purity of the p-H2. Samples
were left overnight to ensure complete mixing. For experiments concerning mixtures of
different isotopes of H2, i.e. p-H2/o-H2 or p-H2/D2, the less concentrated species was
83
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 107 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 4.4. H2 Cluster Formation via Supersonic Co-expansion of H2 and He. In general, He droplets formed by supersonic expansion are doped with atomic or molecular species in a pickup chamber. As it follows from eq. (4.2), CARS signal depends as a square of the number density signifying a 4th power dependence on the CARS signal with increasing distance from the nozzle. Therefore, besides expansion of a liquid jet of neat H2, experiments can only be performed at distances no more than 5 cm away from the nozzle. Using a pickup cell is not practical, as it must be accommodated in the high density range of the expansion and thus will interfere with the expansion. As a result, for CARS experiments, we perform either a supersonic co-expansion of H2 seeded in He or an expansion of neat H2. 4.5. Sample Preparation Parahydrogen was mixed with He in a 1 gal. stainless steel sampling cylinder. p- H2 having o-H2 content of less than 0.01% was prepared by passing liquid n-H2 (research grade) over a paramagnetic material at temperature of about 16 K.5 Sample mixtures were prepared by first purging the sample cylinder with helium gas. Afterward, known amounts of p-H2 were added and subsequently filled up with He gas to pressures no higher than 40 bar. Total pressures of 40 bar were necessary to perform experiments for multiple days, typically never exceeding 3 days use to ensure purity of the p-H2. Samples were left overnight to ensure complete mixing. For experiments concerning mixtures of different isotopes of H2, i.e. p-H2/o-H2 or p-H2/D2, the less concentrated species was 83 |
