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transmitter.
Normal hydrogen (Praxair, research grade) was converted to pH2, with residual
oH2 content of less than 0.1%, by catalytic conversion of liquid H2 at 17 K. A Brooks
5850i mass flow controller was used to ensure sufficient contact time with the catalytic
material. Upon conversion, liquid pH2 was continuously flowed into the optical cell at a
temperature of 15 K. The procedure for preparing the solid crystalline sample follows
that of Hakuta et. al.14 The cell was first filled with liquid pH2, and gradually cooled
down to 9 K under constant pressure of 24 bar. Upon cooling, the crystal grew from the
upper copper surface downward until the entire cell was filled. Once solid, the cell can
be further cooled down to 4 K. Due to the decrease of the molar volume of solid pH2
with temperature, the internal pressure of the solid sample is expected to be close to
zero,14 but cannot be measured in this work.
The spectra of the bulk liquid or solid were obtained via CARS15 technique. In
this four-wave mixing technique, a 532 nm pump beam generated from a Nd:YAG laser
(Powerlite Precision II 8020) and Stokes beam from a dye laser (Lambda Physics, NPD
3000) were focused onto the center of the pH2 sample by means of a 2 meter plano-convex
lens. Both pump and Stokes beams have vertical polarization. When the
frequency difference of the pump and Stokes laser beams matches the vibrational or
rotational frequency of the pH2 molecules, an anti-Stokes signal is generated, which is
separated from the pump and Stokes beams by means of a diffraction grating and
detected thereafter by a pyro-electric detector. The pump beam has a linewidth of about
0.003 cm-1, whereas the Stokes beam has a linewidth of approximately 0.05 cm-1 and 0.25
140
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 164 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | transmitter. Normal hydrogen (Praxair, research grade) was converted to pH2, with residual oH2 content of less than 0.1%, by catalytic conversion of liquid H2 at 17 K. A Brooks 5850i mass flow controller was used to ensure sufficient contact time with the catalytic material. Upon conversion, liquid pH2 was continuously flowed into the optical cell at a temperature of 15 K. The procedure for preparing the solid crystalline sample follows that of Hakuta et. al.14 The cell was first filled with liquid pH2, and gradually cooled down to 9 K under constant pressure of 24 bar. Upon cooling, the crystal grew from the upper copper surface downward until the entire cell was filled. Once solid, the cell can be further cooled down to 4 K. Due to the decrease of the molar volume of solid pH2 with temperature, the internal pressure of the solid sample is expected to be close to zero,14 but cannot be measured in this work. The spectra of the bulk liquid or solid were obtained via CARS15 technique. In this four-wave mixing technique, a 532 nm pump beam generated from a Nd:YAG laser (Powerlite Precision II 8020) and Stokes beam from a dye laser (Lambda Physics, NPD 3000) were focused onto the center of the pH2 sample by means of a 2 meter plano-convex lens. Both pump and Stokes beams have vertical polarization. When the frequency difference of the pump and Stokes laser beams matches the vibrational or rotational frequency of the pH2 molecules, an anti-Stokes signal is generated, which is separated from the pump and Stokes beams by means of a diffraction grating and detected thereafter by a pyro-electric detector. The pump beam has a linewidth of about 0.003 cm-1, whereas the Stokes beam has a linewidth of approximately 0.05 cm-1 and 0.25 140 |
