Page 210 |
Save page Remove page | Previous | 210 of 238 | Next |
|
small (250x250 max)
medium (500x500 max)
Large (1000x1000 max)
Extra Large
large ( > 500x500)
Full Resolution
All (PDF)
|
This page
All
|
observed. Furthermore, due to supersonic expansion in helium droplets, increased
distances provide the additional cooling to ensure that the clusters are at the low
temperature of T = 0.4 K as dictated by the helium droplet. Unfortunately, as discussed
in Chapter 4, rotational CARS experiments could not be performed presently due to
signal constraints. Nevertheless, attempts of rotational CARS should be made once
corresponding issues have been addressed.
An additional problem of probing the clusters at far distances from the expansion
source is the subsequent decrease in number density. This issue can be addressed by
lowering the temperature of the source to create larger clusters. However, operation of
the pulsed valve generates sufficient energy to limit the temperature of the source to
temperatures no lower than TV = 10 K at 1 Hz repetition rate, TV = 12 K at 20 Hz.
Therefore, additional cooling of the pulsed valve is necessary to provide large helium
droplets. For temperatures down to TV ≤ 6 K, recent work with continuous nozzles has
indicated droplets on the order of 108 – 1010 He atoms can be produced. Based on our
current work with pulsed nozzles, it is likely that larger droplets at similar low
temperature conditions can be achieved. For co-expansion of H2 with He, this will
undoubtedly freeze H2 inside the nozzle. However, if such temperatures can be
accomplished, then a pickup cell placed away from the nozzle may provide suitable
means for doping.
For determining superfluidity of H2 clusters, several possible methods exist. For
example, transitions from normal liquid to superfluid may be accompanied by significant
decrease in the linewidth of the spectral lines. For comparison, the superfluid transition
186
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 210 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | observed. Furthermore, due to supersonic expansion in helium droplets, increased distances provide the additional cooling to ensure that the clusters are at the low temperature of T = 0.4 K as dictated by the helium droplet. Unfortunately, as discussed in Chapter 4, rotational CARS experiments could not be performed presently due to signal constraints. Nevertheless, attempts of rotational CARS should be made once corresponding issues have been addressed. An additional problem of probing the clusters at far distances from the expansion source is the subsequent decrease in number density. This issue can be addressed by lowering the temperature of the source to create larger clusters. However, operation of the pulsed valve generates sufficient energy to limit the temperature of the source to temperatures no lower than TV = 10 K at 1 Hz repetition rate, TV = 12 K at 20 Hz. Therefore, additional cooling of the pulsed valve is necessary to provide large helium droplets. For temperatures down to TV ≤ 6 K, recent work with continuous nozzles has indicated droplets on the order of 108 – 1010 He atoms can be produced. Based on our current work with pulsed nozzles, it is likely that larger droplets at similar low temperature conditions can be achieved. For co-expansion of H2 with He, this will undoubtedly freeze H2 inside the nozzle. However, if such temperatures can be accomplished, then a pickup cell placed away from the nozzle may provide suitable means for doping. For determining superfluidity of H2 clusters, several possible methods exist. For example, transitions from normal liquid to superfluid may be accompanied by significant decrease in the linewidth of the spectral lines. For comparison, the superfluid transition 186 |
