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3.6. Electron-Impact Ionization of He Droplets
In order to understand the enhancement of M = 16 signal upon increase of the
droplet size, the mechanism for electron impact ionization and excitation must be
considered. In small droplets, an ionizing collision produces a He+
ion in the droplet with
ejection of both electrons, followed by relaxation of the droplet to produce He2
+ or larger
ions. The energy relaxation and subsequent ejection of He+ ions allows detection of the
charged species. In large droplets, it is believed that electronically excited atoms (He*)
become important. In bulk helium it is known that, upon excitation, He* species will
capture another atom in approximately 15 μs forming metastable a 3Σu
+ (He2
*) excimer
molecules in vibrationally excited states.9 If this formation takes place near the surface
of the droplet, the energy released could be significant to eject the vibrationally excited
He2
*. Upon ejection, the neutral excitations could produce positive ions by collision with
metal surfaces, as observed in bulk helium.10 In addition, a collision of the free He2
* with
a second electron is also possible. In large droplets two He2
* excimers can be produced.
Upon collision they may yield He4
+ ions, which are then ejected.
As obtained by our current work in pulsed nozzles, previous studies6,11 have
observed a delayed cluster feature unaffected by either the accelerating or quadrupole
fields of the mass spectrometer. This delayed feature was attributed to very large ionized
clusters or excited neutral clusters. In bulk helium, He2
* can be formed in the ground
vibrational state by the relaxation of an excited He2
+ ion and its subsequent
recombination with an electron.9 This process of cluster-electron recombination is likely
64
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 88 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 3.6. Electron-Impact Ionization of He Droplets In order to understand the enhancement of M = 16 signal upon increase of the droplet size, the mechanism for electron impact ionization and excitation must be considered. In small droplets, an ionizing collision produces a He+ ion in the droplet with ejection of both electrons, followed by relaxation of the droplet to produce He2 + or larger ions. The energy relaxation and subsequent ejection of He+ ions allows detection of the charged species. In large droplets, it is believed that electronically excited atoms (He*) become important. In bulk helium it is known that, upon excitation, He* species will capture another atom in approximately 15 μs forming metastable a 3Σu + (He2 *) excimer molecules in vibrationally excited states.9 If this formation takes place near the surface of the droplet, the energy released could be significant to eject the vibrationally excited He2 *. Upon ejection, the neutral excitations could produce positive ions by collision with metal surfaces, as observed in bulk helium.10 In addition, a collision of the free He2 * with a second electron is also possible. In large droplets two He2 * excimers can be produced. Upon collision they may yield He4 + ions, which are then ejected. As obtained by our current work in pulsed nozzles, previous studies6,11 have observed a delayed cluster feature unaffected by either the accelerating or quadrupole fields of the mass spectrometer. This delayed feature was attributed to very large ionized clusters or excited neutral clusters. In bulk helium, He2 * can be formed in the ground vibrational state by the relaxation of an excited He2 + ion and its subsequent recombination with an electron.9 This process of cluster-electron recombination is likely 64 |
