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(A) The pulsed nozzle assembly is cooled by a closed cycle refrigerator
(Sumitomo Cryocooler, SRDK–408 DW), and its temperature is controlled by resistive
heating. The pulsed nozzle is attached to a copper block with apiezon grease which is
subsequently attached to the second stage of the closed cycle refrigerator, see Fig. 3.2. In
addition, the stainless steel housing of the valve is wrapped in a copper sheet, which is in
turn in thermal contact with the cold head for additional cooling of the valve (Not shown
in current picture). An aluminum radiation shield is attached to the first stage of the
cryostat in order to avoid radiation heating of the nozzle. An additional, detachable plate
housed on the radiation shield covering the nozzle itself is used for enhanced cooling.
Due to its restrictions, the plate is removed when performing experiments close to the
nozzle, such as CARS experiments, see Chapter 4. The temperature of the pulse nozzle
setup is obtained with two silicon diodes placed on the copper block (TC) and on the
copper sheet attached to the pulsed valve body (TV). The TC sensor is located in the
lower right hand side of Fig. 3.2 while the TV sensor is not shown. We found that
because of the extensive heating due to the operation of the valve, TV is a more accurate
description of the temperature of the expanding gas. However, due to the stainless steel
housing, knowledge of the actual temperature of the expanding gas is not known
accurately.
40
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 64 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | (A) The pulsed nozzle assembly is cooled by a closed cycle refrigerator (Sumitomo Cryocooler, SRDK–408 DW), and its temperature is controlled by resistive heating. The pulsed nozzle is attached to a copper block with apiezon grease which is subsequently attached to the second stage of the closed cycle refrigerator, see Fig. 3.2. In addition, the stainless steel housing of the valve is wrapped in a copper sheet, which is in turn in thermal contact with the cold head for additional cooling of the valve (Not shown in current picture). An aluminum radiation shield is attached to the first stage of the cryostat in order to avoid radiation heating of the nozzle. An additional, detachable plate housed on the radiation shield covering the nozzle itself is used for enhanced cooling. Due to its restrictions, the plate is removed when performing experiments close to the nozzle, such as CARS experiments, see Chapter 4. The temperature of the pulse nozzle setup is obtained with two silicon diodes placed on the copper block (TC) and on the copper sheet attached to the pulsed valve body (TV). The TC sensor is located in the lower right hand side of Fig. 3.2 while the TV sensor is not shown. We found that because of the extensive heating due to the operation of the valve, TV is a more accurate description of the temperature of the expanding gas. However, due to the stainless steel housing, knowledge of the actual temperature of the expanding gas is not known accurately. 40 |
