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348 349 350 351 352 353 354 355 356 357 358
a)
Intensity, a.u.
Wavenumber, cm-1
T = 5 K
T = 22.3 K
T = 9 K b)
T = 13 K c)
T = 15 K d)
T = 18.5 K e)
f)
FIG. 6.1. CARS spectra of the S0(0) transition in both solid (a) – (c) and liquid (d) – (f)
pH2 at different temperatures as measured without an intra-cavity etalon. Measurements
in liquid were obtained at P = 2 bar. The frequency of the S0(0) line in the gas phase at
354.37 cm-1
is shown by a dashed vertical line.20
The spectra of the S0(0) line in liquid pH2 at different temperatures and at a
pressure of 2 bar are shown in traces (d) – (f) of Fig. 6.1. The observation of the single
line is consistent with the previously measured spontaneous Raman spectra in liquid
pH2.11 The absence of the splitting reflects the averaging of the anisotropic
intermolecular interaction in the liquid. The S0(0) line has the same frequency of 353.0 ±
0.1 cm-1 and similar linewidths of approximately 1 cm-1 in the temperature interval of 15
K - 24 K. For comparison, the frequency of the S0(0) line in liquid pH2 of 353.3 cm-1 has
previously been measured at 18 K.11 The frequency of the S0(0) line in the gas phase at
354.37 cm-1
is shown in Fig. 6.1 by a dashed vertical line.20
Typical CARS spectra of the vibrational Q1(0) ( ν = 1 ← 0, J = 0 ← 0 ) line in
142
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 166 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 348 349 350 351 352 353 354 355 356 357 358 a) Intensity, a.u. Wavenumber, cm-1 T = 5 K T = 22.3 K T = 9 K b) T = 13 K c) T = 15 K d) T = 18.5 K e) f) FIG. 6.1. CARS spectra of the S0(0) transition in both solid (a) – (c) and liquid (d) – (f) pH2 at different temperatures as measured without an intra-cavity etalon. Measurements in liquid were obtained at P = 2 bar. The frequency of the S0(0) line in the gas phase at 354.37 cm-1 is shown by a dashed vertical line.20 The spectra of the S0(0) line in liquid pH2 at different temperatures and at a pressure of 2 bar are shown in traces (d) – (f) of Fig. 6.1. The observation of the single line is consistent with the previously measured spontaneous Raman spectra in liquid pH2.11 The absence of the splitting reflects the averaging of the anisotropic intermolecular interaction in the liquid. The S0(0) line has the same frequency of 353.0 ± 0.1 cm-1 and similar linewidths of approximately 1 cm-1 in the temperature interval of 15 K - 24 K. For comparison, the frequency of the S0(0) line in liquid pH2 of 353.3 cm-1 has previously been measured at 18 K.11 The frequency of the S0(0) line in the gas phase at 354.37 cm-1 is shown in Fig. 6.1 by a dashed vertical line.20 Typical CARS spectra of the vibrational Q1(0) ( ν = 1 ← 0, J = 0 ← 0 ) line in 142 |
