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distinguish ions produced via direct ionization from those ejected upon vibrational
excitation. This fragmentation induced background can be removed by employing
electron-impact ionization at distances far away from the quadrupole and ion optics such
that the unwanted ionized fragments are not mass selected. In addition, large enough
droplets containing ions of interest must survive the fragmentation process for
experiments to be performed. Fortunately, as discussed in Chapter 3, droplets obtained
from our pulsed nozzle are sufficiently large that a fraction of ionized droplets are not
deflected by the electric fields in the QMS, see Fig. 3.4. As a result, we can utilize these
large ionized droplets for the proposed experiments.
We have previously observed that He droplets can be ionized using an electron-gun
(EGUN) installed on our TOF-MS, which is housed approximately 10 cm upstream
from the entrance to the QMS. In addition, only the un-deflected large droplets
containing ions will be able to reach the QMS region, while all lighter ions can be
retarded by the electric field. As a result, installation of another ionizer is not necessary.
For preliminary experiments, the simplest molecule to study for this project is
water. Mass spectra of He droplets (see Fig. 3.3) indicate sizeable H2O+ peaks, which
originate from pickup of residual water gas. As a result, no pickup cell is necessary. Fig.
8.2 shows vibrational spectra for a variety of protonated water clusters, H+(H2O)N, for N
= 2 - 11. For clusters larger than N = 3, the bands are significantly broadened. Due to
the spectral broadening, water clusters should provide easier IR tuning in which the laser
wavelength can be centered only approximately at the maximum.
191
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 215 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | distinguish ions produced via direct ionization from those ejected upon vibrational excitation. This fragmentation induced background can be removed by employing electron-impact ionization at distances far away from the quadrupole and ion optics such that the unwanted ionized fragments are not mass selected. In addition, large enough droplets containing ions of interest must survive the fragmentation process for experiments to be performed. Fortunately, as discussed in Chapter 3, droplets obtained from our pulsed nozzle are sufficiently large that a fraction of ionized droplets are not deflected by the electric fields in the QMS, see Fig. 3.4. As a result, we can utilize these large ionized droplets for the proposed experiments. We have previously observed that He droplets can be ionized using an electron-gun (EGUN) installed on our TOF-MS, which is housed approximately 10 cm upstream from the entrance to the QMS. In addition, only the un-deflected large droplets containing ions will be able to reach the QMS region, while all lighter ions can be retarded by the electric field. As a result, installation of another ionizer is not necessary. For preliminary experiments, the simplest molecule to study for this project is water. Mass spectra of He droplets (see Fig. 3.3) indicate sizeable H2O+ peaks, which originate from pickup of residual water gas. As a result, no pickup cell is necessary. Fig. 8.2 shows vibrational spectra for a variety of protonated water clusters, H+(H2O)N, for N = 2 - 11. For clusters larger than N = 3, the bands are significantly broadened. Due to the spectral broadening, water clusters should provide easier IR tuning in which the laser wavelength can be centered only approximately at the maximum. 191 |
