Page 163 |
Save page Remove page | Previous | 163 of 238 | Next |
|
small (250x250 max)
medium (500x500 max)
Large (1000x1000 max)
Extra Large
large ( > 500x500)
Full Resolution
All (PDF)
|
This page
All
|
used as a monitor of the temperature of the clusters. Surprisingly, we did not find any
systematic study of the temperature dependence of the Raman spectra in the liquid,
except for a single data point at 18 K dating back to 1962.11 Therefore, this work is
aimed at studying the temperature dependence of the vibrational and rotational spectra in
liquid pH2. We have observed that the vibrational frequency of pH2 liquid increases by
about 2 cm-1 upon change of temperature from 14 K to 30 K. We have found that the
frequency change, Δν, scales as the square of the density of the liquid, ρL. The obtained
scaling enables prediction of the vibrational frequency in the liquid below the freezing
temperature.
6.2. Experimental Technique
The cryogenic experimental apparatus is described in detail elsewhere.12 The
system consists of a Janis SHI-4 optical cryostat equipped with a Sumitomo RDK-408D
closed-cycle refrigerator. The 3-cm long optical cell is made of stainless steel and has a 1
× 1 cm2 optical clearance and two CaF2 windows. The upper part of the cell has a copper
post, which is directly attached to the second stage of the refrigerator through a copper
heat conductor. The cell is screened from thermal radiation by an aluminum shield
attached to the first stage of the refrigerator and is mounted inside a vacuum shroud,
which is also equipped with CaF2 windows. The cell temperature can be varied in the
range of 4 to 30 K by resistive heating of the second stage of the refrigerator. The
temperature of the liquid hydrogen inside the cell was calibrated via known saturated
vapor pressure for hydrogen13 as measured by means of a Wika TRONIC pressure
139
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 163 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | used as a monitor of the temperature of the clusters. Surprisingly, we did not find any systematic study of the temperature dependence of the Raman spectra in the liquid, except for a single data point at 18 K dating back to 1962.11 Therefore, this work is aimed at studying the temperature dependence of the vibrational and rotational spectra in liquid pH2. We have observed that the vibrational frequency of pH2 liquid increases by about 2 cm-1 upon change of temperature from 14 K to 30 K. We have found that the frequency change, Δν, scales as the square of the density of the liquid, ρL. The obtained scaling enables prediction of the vibrational frequency in the liquid below the freezing temperature. 6.2. Experimental Technique The cryogenic experimental apparatus is described in detail elsewhere.12 The system consists of a Janis SHI-4 optical cryostat equipped with a Sumitomo RDK-408D closed-cycle refrigerator. The 3-cm long optical cell is made of stainless steel and has a 1 × 1 cm2 optical clearance and two CaF2 windows. The upper part of the cell has a copper post, which is directly attached to the second stage of the refrigerator through a copper heat conductor. The cell is screened from thermal radiation by an aluminum shield attached to the first stage of the refrigerator and is mounted inside a vacuum shroud, which is also equipped with CaF2 windows. The cell temperature can be varied in the range of 4 to 30 K by resistive heating of the second stage of the refrigerator. The temperature of the liquid hydrogen inside the cell was calibrated via known saturated vapor pressure for hydrogen13 as measured by means of a Wika TRONIC pressure 139 |
