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xiv
FIG. 3.3. 45
Mass spectrum obtained via pulsed gas expansion of pure helium and detected by
quadrupole mass spectrometer. The signal was recorded using a boxcar integrator set at
delay of 3.2 ms and Δt = 3 μs. The large intensity peak in the range of M = 17 – 19 stems
from water molecules and dimers trapped in He droplets.
FIG. 3.4. 46
Temporal profile of the droplet signal as measured at m/z = 4 and 8. TV = 16 K, P0 = 15
bar at 200 μs nominal pulse duration and 20 Hz repetition rate.
FIG. 3.5. 48
Temperature dependence (TV) of m/z = 8 signal using pulsed nozzle at a repetition rate of
20 Hz and nominal pulse width of 200 μs.
FIG. 3.6. 49
Temperature dependence (TV) of time-delayed signal at t = 4.0 ms for M = 8 at P0 = 15
bar, 200 μs pulse duration and various pulse repetition rates.
FIG. 3.7. 50
Temperature dependence on the velocity of the droplets for pulsed and continuous
nozzles at several operating conditions for M = 8 amu. Solid line indicates estimated
velocity of ideal gas as calculated using gas kinetic theory.
FIG. 3.8. 51
m/z = 8 signal dependence vs. nominal pulse duration at TV = 16 K, P0 = 20 bar, and 20
Hz repetition rate.
FIG. 3.9. 52
m/z = 8 temporal profiles at various pulse durations (a – c) at TV = 16 K, P0 = 20 bar, and
20 Hz repetition rate.
FIG. 3.10. 53
Pressure in nozzle and QMS chambers vs. pulse duration at TV = 17 K, P0 = 20 bar, and
20 Hz repetition rate. QMS pressure is factored by 103 for scaling purposes.
FIG. 3.11. 54
m/z = 8 signal vs. He backing pressure at TV = 20 K, 200 μs pulse duration, and 20 Hz
repetition rate.
FIG. 3.12. 55
Dependence of the pressure rise in the nozzle and QMS chambers vs. nozzle He backing
pressure at TV = 20 K, 200 μs pulse duration, and 20 Hz repetition rate. The QMS
pressure is scaled by 2·102 for comparison.
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 14 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | xiv FIG. 3.3. 45 Mass spectrum obtained via pulsed gas expansion of pure helium and detected by quadrupole mass spectrometer. The signal was recorded using a boxcar integrator set at delay of 3.2 ms and Δt = 3 μs. The large intensity peak in the range of M = 17 – 19 stems from water molecules and dimers trapped in He droplets. FIG. 3.4. 46 Temporal profile of the droplet signal as measured at m/z = 4 and 8. TV = 16 K, P0 = 15 bar at 200 μs nominal pulse duration and 20 Hz repetition rate. FIG. 3.5. 48 Temperature dependence (TV) of m/z = 8 signal using pulsed nozzle at a repetition rate of 20 Hz and nominal pulse width of 200 μs. FIG. 3.6. 49 Temperature dependence (TV) of time-delayed signal at t = 4.0 ms for M = 8 at P0 = 15 bar, 200 μs pulse duration and various pulse repetition rates. FIG. 3.7. 50 Temperature dependence on the velocity of the droplets for pulsed and continuous nozzles at several operating conditions for M = 8 amu. Solid line indicates estimated velocity of ideal gas as calculated using gas kinetic theory. FIG. 3.8. 51 m/z = 8 signal dependence vs. nominal pulse duration at TV = 16 K, P0 = 20 bar, and 20 Hz repetition rate. FIG. 3.9. 52 m/z = 8 temporal profiles at various pulse durations (a – c) at TV = 16 K, P0 = 20 bar, and 20 Hz repetition rate. FIG. 3.10. 53 Pressure in nozzle and QMS chambers vs. pulse duration at TV = 17 K, P0 = 20 bar, and 20 Hz repetition rate. QMS pressure is factored by 103 for scaling purposes. FIG. 3.11. 54 m/z = 8 signal vs. He backing pressure at TV = 20 K, 200 μs pulse duration, and 20 Hz repetition rate. FIG. 3.12. 55 Dependence of the pressure rise in the nozzle and QMS chambers vs. nozzle He backing pressure at TV = 20 K, 200 μs pulse duration, and 20 Hz repetition rate. The QMS pressure is scaled by 2·102 for comparison. |
