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Multiplier Tube EMI 9789QA. The voltage supplied to the PMT can be varied from 0 –
2000 V. However, to avoid saturation in the CARS spectra, the voltage was set typically
at 700 V. In addition, the CARS signal was amplified by a fast pre-amplifier and
recorded by a Boxcar Integrator. The trigger signal for the Boxcar is supplied by a
photodiode exposed to the scattered light of the pump laser. Neutral filters were used at
the entrance of the PMT to avoid saturation effects.
The averaged CARS signal supplied by the Boxcar was recorded using a data
acquisition card (PCI-MIO E card from National Instruments) and controlled by a
LabView application. During the scanning mode of the dye laser, a 0 – 10 V linear ramp
was supplied from the dye laser corresponding to the starting and ending points of the
scan. As a result, the obtained average signal can be associated with the corresponding
wavelength. Calibration of the dye laser is obtained by comparison to the known Raman
spectrum of H2 gas.6
4.7. Vibrational CARS of H2 Molecules and Clusters in He Droplets
A typical spectrum of n-H2 expanded at TV = 300 K is shown in Fig. 4.6. The
spectrum was calibrated based on the known line positions in the gas phase.6 In Fig. 4.6,
the ν = 1, J = 1 ← ν = 0, J = 1 o-H2 transition, Q1(1), is observed around 4155.30 cm-1
while the p-H2 Q1(0) transition (ν = 1, J = 0 ← ν = 0, J = 0) is blue-shifted at 4161.17
cm-1. The intensity ratio of the two lines was found to be 8.7 ± 0.5 which corresponds to
an ortho-para ratio (OPR) of approximately 3:1, in accordance with the known ratio at
85
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 109 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | Multiplier Tube EMI 9789QA. The voltage supplied to the PMT can be varied from 0 – 2000 V. However, to avoid saturation in the CARS spectra, the voltage was set typically at 700 V. In addition, the CARS signal was amplified by a fast pre-amplifier and recorded by a Boxcar Integrator. The trigger signal for the Boxcar is supplied by a photodiode exposed to the scattered light of the pump laser. Neutral filters were used at the entrance of the PMT to avoid saturation effects. The averaged CARS signal supplied by the Boxcar was recorded using a data acquisition card (PCI-MIO E card from National Instruments) and controlled by a LabView application. During the scanning mode of the dye laser, a 0 – 10 V linear ramp was supplied from the dye laser corresponding to the starting and ending points of the scan. As a result, the obtained average signal can be associated with the corresponding wavelength. Calibration of the dye laser is obtained by comparison to the known Raman spectrum of H2 gas.6 4.7. Vibrational CARS of H2 Molecules and Clusters in He Droplets A typical spectrum of n-H2 expanded at TV = 300 K is shown in Fig. 4.6. The spectrum was calibrated based on the known line positions in the gas phase.6 In Fig. 4.6, the ν = 1, J = 1 ← ν = 0, J = 1 o-H2 transition, Q1(1), is observed around 4155.30 cm-1 while the p-H2 Q1(0) transition (ν = 1, J = 0 ← ν = 0, J = 0) is blue-shifted at 4161.17 cm-1. The intensity ratio of the two lines was found to be 8.7 ± 0.5 which corresponds to an ortho-para ratio (OPR) of approximately 3:1, in accordance with the known ratio at 85 |
