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magnitude of the pulses. Due to dark noise at high applied multiplier voltage, a small
number of counts are still observed even in the absence of He droplets. This noise is due
to the increased level of spontaneous electron emission at high voltages as well as due to
some ionization of the background gas in the detection chamber as well as cosmic rays.
In addition, the higher the electron multiplier voltage, the larger the rate of the
background counts. As a result, the right-hand side panels of Fig. 2.4, (e) – (h), represent
pulse counts with the direct He beam blocked from reaching the ionizer by half-closing
valve GV2. Using GV2 allows us to account for the background pressure due to He gas.
Panels (a) – (d) represent total signal from both He droplets and background.
As shown in Fig. 2.4 for the case of attenuation by 100 μm pinhole, some of the
pulses are overlapped with other pulses, which may influence the statistics of the signal.
Therefore the flux was further reduced by employing a 35 μm orifice, as shown in Fig.
2.5, which shows no pulse overlaps. Here the total number of counts is reduced due to a
factor of about 8 smaller cross section of the 35 μm pinhole. Comparison between panels
(d) and (h) in Fig. 2.5 at T0 = 6.5 K for droplets and background, respectively, show
similar count amplitudes. The pulses observed in panel (h) are due to single electrons, in
which the intensity distribution reflects the different level of amplification of the
electrons.
19
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 43 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | magnitude of the pulses. Due to dark noise at high applied multiplier voltage, a small number of counts are still observed even in the absence of He droplets. This noise is due to the increased level of spontaneous electron emission at high voltages as well as due to some ionization of the background gas in the detection chamber as well as cosmic rays. In addition, the higher the electron multiplier voltage, the larger the rate of the background counts. As a result, the right-hand side panels of Fig. 2.4, (e) – (h), represent pulse counts with the direct He beam blocked from reaching the ionizer by half-closing valve GV2. Using GV2 allows us to account for the background pressure due to He gas. Panels (a) – (d) represent total signal from both He droplets and background. As shown in Fig. 2.4 for the case of attenuation by 100 μm pinhole, some of the pulses are overlapped with other pulses, which may influence the statistics of the signal. Therefore the flux was further reduced by employing a 35 μm orifice, as shown in Fig. 2.5, which shows no pulse overlaps. Here the total number of counts is reduced due to a factor of about 8 smaller cross section of the 35 μm pinhole. Comparison between panels (d) and (h) in Fig. 2.5 at T0 = 6.5 K for droplets and background, respectively, show similar count amplitudes. The pulses observed in panel (h) are due to single electrons, in which the intensity distribution reflects the different level of amplification of the electrons. 19 |
