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Chapter VIII. Conclusions and Future Works
8.1. Conclusions
In this work we have studied different molecular systems at ultra-cold
temperatures, such as in Ar matrices, in He droplets and in hydrogen clusters. Para-H2
clusters embedded in helium droplets have been studied via coherent anti-stokes Raman
spectroscopy. Based on our Raman spectroscopy study in bulk p-H2, we have been able
to determine an upper limit to the temperature of liquid p-H2 clusters to ≤ 5 K.
Furthermore, our bulk p-H2 work has provided the first systematic study on the
temperature dependence of the vibrational Q1(0) line and therefore provides a benchmark
for future experiments and calculations. In addition, we have discovered phase
separation in mixtures of p-H2/D2 clusters. By employing a simple thermodynamic
model of phase separation, we have estimated the temperatures of the mixed p-H2/D2
clusters to be ≤ 3 K, providing additional verification on the supercooled liquid state of
the H2 clusters.
A significant portion of this thesis is devoted to the characterization of helium
droplets obtained in pulsed nozzle beam expansions, which is much less characterized as
compared to its continuous nozzle counterpart. As a result, we have summarized the
optimization of parameters dictating cluster formation. Based on a collaborative work
using cw nozzles, we have been able to provide a method for determining droplet size
and their corresponding size distributions.
184
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 208 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | Chapter VIII. Conclusions and Future Works 8.1. Conclusions In this work we have studied different molecular systems at ultra-cold temperatures, such as in Ar matrices, in He droplets and in hydrogen clusters. Para-H2 clusters embedded in helium droplets have been studied via coherent anti-stokes Raman spectroscopy. Based on our Raman spectroscopy study in bulk p-H2, we have been able to determine an upper limit to the temperature of liquid p-H2 clusters to ≤ 5 K. Furthermore, our bulk p-H2 work has provided the first systematic study on the temperature dependence of the vibrational Q1(0) line and therefore provides a benchmark for future experiments and calculations. In addition, we have discovered phase separation in mixtures of p-H2/D2 clusters. By employing a simple thermodynamic model of phase separation, we have estimated the temperatures of the mixed p-H2/D2 clusters to be ≤ 3 K, providing additional verification on the supercooled liquid state of the H2 clusters. A significant portion of this thesis is devoted to the characterization of helium droplets obtained in pulsed nozzle beam expansions, which is much less characterized as compared to its continuous nozzle counterpart. As a result, we have summarized the optimization of parameters dictating cluster formation. Based on a collaborative work using cw nozzles, we have been able to provide a method for determining droplet size and their corresponding size distributions. 184 |
