Page 41 |
Save page Remove page | Previous | 41 of 238 | Next |
|
small (250x250 max)
medium (500x500 max)
Large (1000x1000 max)
Extra Large
large ( > 500x500)
Full Resolution
All (PDF)
|
This page
All
|
lasts for a few nanoseconds. Thus, a series of pulses are generated during the flight of the
droplet through the ionizer. Due to the slow response time of the pre-amplifier of about
10 μs, these individual pulses cannot been resolved As a result, the signal from one
droplet produces a pulse having width of about 100 μs. The attenuation of the droplet
beam eliminates the effects of simultaneous detection of multiple droplets.
The droplet pulses have been recorded by a fast digitizer (National Instruments
PCI-MIO-16E-4) having resolution of 10 μs per channel. A LabView data acquisition
program is employed to record the total events that occur. Large time segments of about
10 seconds each have been recorded to ensure sufficient number of detected pulses. As a
result, several thousands of events were recorded each time.
2.3.4. Average He Droplet Sizes
Fig. 2.3 illustrates results10 of the average helium droplet size for various
temperatures based on the attenuation of the He droplet beam upon multiple collisions
with Ar and He atoms at room temperature, as shown by solid squares and triangles,
respectively. These results were measured in our laboratory by Luis Gomez and are
presented here for convenience. Detailed discussion on the average droplet size can be
obtained elsewhere.10 Previous deflection results are shown by stars.6,9,11
17
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 41 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | lasts for a few nanoseconds. Thus, a series of pulses are generated during the flight of the droplet through the ionizer. Due to the slow response time of the pre-amplifier of about 10 μs, these individual pulses cannot been resolved As a result, the signal from one droplet produces a pulse having width of about 100 μs. The attenuation of the droplet beam eliminates the effects of simultaneous detection of multiple droplets. The droplet pulses have been recorded by a fast digitizer (National Instruments PCI-MIO-16E-4) having resolution of 10 μs per channel. A LabView data acquisition program is employed to record the total events that occur. Large time segments of about 10 seconds each have been recorded to ensure sufficient number of detected pulses. As a result, several thousands of events were recorded each time. 2.3.4. Average He Droplet Sizes Fig. 2.3 illustrates results10 of the average helium droplet size for various temperatures based on the attenuation of the He droplet beam upon multiple collisions with Ar and He atoms at room temperature, as shown by solid squares and triangles, respectively. These results were measured in our laboratory by Luis Gomez and are presented here for convenience. Detailed discussion on the average droplet size can be obtained elsewhere.10 Previous deflection results are shown by stars.6,9,11 17 |
