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Chapter I. General Introduction
1.1. Scope of Work
This thesis can be separated into projects focused into four major areas. Although
distinct, each project highlights the use of matrix isolation as an advantageous tool for
studying molecules and molecular aggregates. In addition, by coupling matrix isolation
with rotational and vibrational spectroscopy, both fundamental and practical information
about these systems can be obtained.
The first project is devoted to measuring He droplet size distributions for droplets
formed at cw source temperatures in the range of 5 – 7 K. Due to experimental
difficulties, this temperature range was not covered by previous measurements, so that
the droplet size distribution was unknown before this study. Since the He droplet size is
a necessary parameter for the spectroscopy of molecules and molecular aggregates
embedded inside the droplets, detailed knowledge of droplets in the 5 – 7 K range is of
critical importance for the interpretation of the experimental results. Here we have
developed a novel technique for measuring droplet size distributions and report first
results.
The second project is a continuation of our previous study1,2 of liquid hydrogen
clusters at very low temperature, such as supercooled para-H2 clusters. In this work, we
have discovered a phase separation in cold liquid p-H2/D2 clusters isolated in He droplets
via vibrational CARS spectroscopy of the p-H2 molecules in the cluster. Based on our
1
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 25 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | Chapter I. General Introduction 1.1. Scope of Work This thesis can be separated into projects focused into four major areas. Although distinct, each project highlights the use of matrix isolation as an advantageous tool for studying molecules and molecular aggregates. In addition, by coupling matrix isolation with rotational and vibrational spectroscopy, both fundamental and practical information about these systems can be obtained. The first project is devoted to measuring He droplet size distributions for droplets formed at cw source temperatures in the range of 5 – 7 K. Due to experimental difficulties, this temperature range was not covered by previous measurements, so that the droplet size distribution was unknown before this study. Since the He droplet size is a necessary parameter for the spectroscopy of molecules and molecular aggregates embedded inside the droplets, detailed knowledge of droplets in the 5 – 7 K range is of critical importance for the interpretation of the experimental results. Here we have developed a novel technique for measuring droplet size distributions and report first results. The second project is a continuation of our previous study1,2 of liquid hydrogen clusters at very low temperature, such as supercooled para-H2 clusters. In this work, we have discovered a phase separation in cold liquid p-H2/D2 clusters isolated in He droplets via vibrational CARS spectroscopy of the p-H2 molecules in the cluster. Based on our 1 |
