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somewhat different. However, it is the integral of the peaks which is representative of
the droplet size. As a result, Fig. 2.9 illustrates the normalized dependence of the
integrated area per unit amplitude (A/I) as a function of the amplitude (I) for a series of
small, medium, and large amplitude peaks. From these results, larger peaks can be
corrected for their increased width in which the area of the peaks can be obtained. Due to
the large size of the droplets, we have assumed that the detected ionization signal scales
with the geometrical cross-section of the droplet as (NHe)2/3. As a result, we can assign
each count to a droplet size based on the area of the peak.
-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.70
0.75
0.80
0.85
0.90
0.95
1.00
1.05
A/I
Peak Amplitude, V
A/I = 0.99 -0.26*exp(-1.48*I)
FIG. 2.9. Ratio of integrated peak area to peak amplitude (A/I) vs. peak amplitude for a
variety of peaks as obtained at T0 = 6.0 K and P0 = 20 bar. The solid line is an
approximate fit and is given numerically in the lower right-hand side of the figure.
25
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 49 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | somewhat different. However, it is the integral of the peaks which is representative of the droplet size. As a result, Fig. 2.9 illustrates the normalized dependence of the integrated area per unit amplitude (A/I) as a function of the amplitude (I) for a series of small, medium, and large amplitude peaks. From these results, larger peaks can be corrected for their increased width in which the area of the peaks can be obtained. Due to the large size of the droplets, we have assumed that the detected ionization signal scales with the geometrical cross-section of the droplet as (NHe)2/3. As a result, we can assign each count to a droplet size based on the area of the peak. -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.05 A/I Peak Amplitude, V A/I = 0.99 -0.26*exp(-1.48*I) FIG. 2.9. Ratio of integrated peak area to peak amplitude (A/I) vs. peak amplitude for a variety of peaks as obtained at T0 = 6.0 K and P0 = 20 bar. The solid line is an approximate fit and is given numerically in the lower right-hand side of the figure. 25 |
