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approximately 100 μm diameter, as prepared by focused laser burning and installed on a
custom mount, was inserted at the focal point of the 50 cm achromatic lens inside the
machine at the nozzle beam axis. Because of the contraction of the cold head and nozzle
with decreasing temperature by about 1 – 2 mm, the nozzle shifts upwards as compared
to its position at room temperature during operation. To account for this discrepancy, the
system was cooled to operating conditions at which the nozzle was aligned using a
telescope. Afterward, the custom pinhole was installed at room temperature at the noted
position by the telescope. From here, the pump and Stokes beams can be aligned
individually through the pinhole in which the throughput intensity was maximized using
a power meter.
Upon this initial alignment, a 26-cm long, 5-cm diameter copper gas cell filled
with 0.5 atm of n-H2 gas and equipped with 3 mm thick BK7 windows was installed
inside the helium droplet apparatus on the beam path. The pre-aligned pump and Stokes
beams usually produced sufficient CARS signal visible to the naked eye and was used
for fine tuning of the optical setup. The CARS signal generated from this gas cell was
then used to align into the PMT.
Upon alignment, the gas cell was removed and CARS signal from the pulsed
nozzle could be observed. The remaining chapter discusses the optimization and
characterization of the CARS signal of clusters at various compositions of the expanding
gas containing different fractions of H2 and He.
82
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 106 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | approximately 100 μm diameter, as prepared by focused laser burning and installed on a custom mount, was inserted at the focal point of the 50 cm achromatic lens inside the machine at the nozzle beam axis. Because of the contraction of the cold head and nozzle with decreasing temperature by about 1 – 2 mm, the nozzle shifts upwards as compared to its position at room temperature during operation. To account for this discrepancy, the system was cooled to operating conditions at which the nozzle was aligned using a telescope. Afterward, the custom pinhole was installed at room temperature at the noted position by the telescope. From here, the pump and Stokes beams can be aligned individually through the pinhole in which the throughput intensity was maximized using a power meter. Upon this initial alignment, a 26-cm long, 5-cm diameter copper gas cell filled with 0.5 atm of n-H2 gas and equipped with 3 mm thick BK7 windows was installed inside the helium droplet apparatus on the beam path. The pre-aligned pump and Stokes beams usually produced sufficient CARS signal visible to the naked eye and was used for fine tuning of the optical setup. The CARS signal generated from this gas cell was then used to align into the PMT. Upon alignment, the gas cell was removed and CARS signal from the pulsed nozzle could be observed. The remaining chapter discusses the optimization and characterization of the CARS signal of clusters at various compositions of the expanding gas containing different fractions of H2 and He. 82 |
