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xii
List of Figures
FIG. 2.1. 13
Average He droplet size and mean diameter measured at different stagnation pressures as
a function of nozzle temperature. Red dots correspond to typical expansion conditions of
P0 = 20 bar.Schematic diagram of the He droplet apparatus. See text for details.
FIG. 2.2. 16
Schematic of He droplet apparatus. NZ – cold nozzle, SK –skimmer, PC1 and PC2 –
pickup cells, BG – Baratron vacuum gauge, IG1 and IG2 – ion gauges, SH1 and SH2 –
beam shutters, A1 and A2 – 6 mm diameter apertures, GV1 and GV2 – gate valves, EI –
electron impact ionizer, IB – ion bender, QMS – quadruple mass spectrometer.
FIG. 2.3. 18
Average He droplet size obtained at varying nozzle temperatures from a continuous
expansion of He gas at a stagnation pressure of 20 bars and through a 5 μm nozzle. The
results of the titration measurements with Ar and He gas are shown by solid squares and
triangles, respectively, as obtained previously. Results of previous deflection
measurements are shown by stars. Corresponding droplet diameters (nm) are given on
the right-hand side.
FIG. 2.4. 20
10 ms segment of signal for a variety of source temperatures at P0 = 20 bar and electron
multiplier voltage of 2400 V. The results were obtained using a 100 μm pinhole. Panels
(a – d) represent pulse counts for He droplets while panels (e – h) represent background
counts where direct droplet beam was blocked.
FIG. 2.5. 21
100 ms segment of signal for a variety of source temperatures at P0 = 20 bar and electron
multiplier voltage of 2400 V. The results were obtained using a 35 μm pinhole. Panels
(a – d) represent pulse counts for He droplets while panels (e – h) represent background
counts where direct droplet beam was blocked.
FIG. 2.6. 22
20 ms segment of signal for a variety of source temperatures at P0 = 20 bar and electron
multiplier voltage of 2300 V. All panels were obtained using a 100 μm pinhole. Panels
(a – d) represent pulse counts for He droplets while panels (e – h) represent background
counts where direct droplet beam was blocked.
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 12 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | xii List of Figures FIG. 2.1. 13 Average He droplet size and mean diameter measured at different stagnation pressures as a function of nozzle temperature. Red dots correspond to typical expansion conditions of P0 = 20 bar.Schematic diagram of the He droplet apparatus. See text for details. FIG. 2.2. 16 Schematic of He droplet apparatus. NZ – cold nozzle, SK –skimmer, PC1 and PC2 – pickup cells, BG – Baratron vacuum gauge, IG1 and IG2 – ion gauges, SH1 and SH2 – beam shutters, A1 and A2 – 6 mm diameter apertures, GV1 and GV2 – gate valves, EI – electron impact ionizer, IB – ion bender, QMS – quadruple mass spectrometer. FIG. 2.3. 18 Average He droplet size obtained at varying nozzle temperatures from a continuous expansion of He gas at a stagnation pressure of 20 bars and through a 5 μm nozzle. The results of the titration measurements with Ar and He gas are shown by solid squares and triangles, respectively, as obtained previously. Results of previous deflection measurements are shown by stars. Corresponding droplet diameters (nm) are given on the right-hand side. FIG. 2.4. 20 10 ms segment of signal for a variety of source temperatures at P0 = 20 bar and electron multiplier voltage of 2400 V. The results were obtained using a 100 μm pinhole. Panels (a – d) represent pulse counts for He droplets while panels (e – h) represent background counts where direct droplet beam was blocked. FIG. 2.5. 21 100 ms segment of signal for a variety of source temperatures at P0 = 20 bar and electron multiplier voltage of 2400 V. The results were obtained using a 35 μm pinhole. Panels (a – d) represent pulse counts for He droplets while panels (e – h) represent background counts where direct droplet beam was blocked. FIG. 2.6. 22 20 ms segment of signal for a variety of source temperatures at P0 = 20 bar and electron multiplier voltage of 2300 V. All panels were obtained using a 100 μm pinhole. Panels (a – d) represent pulse counts for He droplets while panels (e – h) represent background counts where direct droplet beam was blocked. |
