Page 1 |
Save page Remove page | Previous | 1 of 122 | Next |
|
small (250x250 max)
medium (500x500 max)
large ( > 500x500)
Full Resolution
All (PDF)
|
This page
All
Subset |
LOCATION, IONIZATION AND SHIELDING OF IMPURITIES
IN HELIUM NANODROPLETS
by
Yanfei Ren
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In partial Fulfillment of the
Requirement for the Degree
DOCTOR OF PHILOSOPHY
(PHYSICS)
December 2007
Copyright 2007 Yanfei Ren
Object Description
| Title | Location, ionization and shielding of impurities in helium nanodroplets |
| Author | Ren, Yanfei |
| Author email | yren@usc.edu |
| Degree | Doctor of Philosophy |
| Document type | Dissertation |
| Degree program | Physics |
| School | College of Letters, Arts and Sciences |
| Date defended/completed | 2007-10-24 |
| Date submitted | 2007 |
| Restricted until | Unrestricted |
| Date published | 2007-11-16 |
| Advisor (committee chair) | Kresin, Vitaly |
| Advisor (committee member) |
Däppen, Werner Zarnadi, Paolo Vilesov, Andrey Bolzler, Hans |
| Abstract | This dissertation is devoted to an experimental investigation of the location and ionization of certain impurities picked up by a helium nanodroplet beam. Supersonic beams of 4He clusters are produced by the low-temperature supersonic expansion method, and the pick-up technique is employed to dope them with impurities. Subsequently, the doped helium droplets are detected and analyzed by a quadrupole mass spectrometer using electron bombardment ionization.; One finding is related to fragmentation caused by electron-impact ionization of amino acid molecules embedded in helium nanodroplets. While molecules embedded individually fragment as extensively as free ones in the gas phase, when they are picked up right after, or right before,a small amount of water, there arises a striking "buffering" effect on the ionization-induced fragmentation. In fact, the fragmentation of glycine is entirely quenched; for tryptophan the effect is weaker but also noticeable. An accompanying experiment involving DCl molecules co-embedded with water showed that Cl and/or DCl invariably leave the droplet upon ionization.; The study was extended to the case of pick-up of glycine oligomers (di-, tri-, and tetraglycine) with water molecules. We found that while the aforementioned "buffering" effect remains qualitatively present for diglycine through tetraglycine, it becomes quantitatively weaker with increasing peptide size. The fragmentation patterns were observed to be robust and insensitive to variations in either the mean droplet size or the electron bombardment energy.; As a complementary investigation, alkane (propane and hexane) and corresponding alkanethiol (1-propanethiol, 1-hexanethiol) also were embedded in the nanodroplets, individually as well as jointly with water. In the former case, the alkanes and alkanethiols were found to fragment the same way as in the gas phase; in the latter case, it was found that the presence of water significantly reduces bond cleavage for alkanes, but has no effect for alkanthiols. It is hypothesized that the observed patterns may be due to the large difference of electric dipole moments between the symmetric alkane molecules and asymmetric alkanthiol molecules, whichresults in preferential steering of the ionizing He+ hole towards the polar molecule. The same effect may contribute to the "buffering" effect of water on amino acid fragmentation as well.; Further, we have measured the electron energy dependence of the ionization yield of picked-up alkaline-earth-metal atoms Ca, Sr, Ba, and Mg. There is a qualitative shape difference between the yield curves of species solvated in the middle of helium droplet and species located in the surface region. The measurements demonstrate that all of the above atoms reside at or near the droplet surface. This work resolves a long-standing experimental and theoretical uncertainty concerning the degree of solvation of alkaline-earth-metal atoms, especially Mg, in 4He nanodroplets. |
| Keyword | helium droplets; impurities; ionization |
| 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 |
| Type | texts |
| Legacy record ID | usctheses-m929 |
| Rights | Ren, Yanfei |
| Repository name | Libraries, University of Southern California |
| Repository address | Los Angeles, California |
| Repository email | http://www.usc.edu/isd/libraries/services/ask_a_librarian/email/ |
| Filename | etd-Ren-20071116 |
| Archival file | uscthesesreloadpub_Volume62/etd-Ren-20071116.pdf |
Description
| Title | Page 1 |
| Full text | LOCATION, IONIZATION AND SHIELDING OF IMPURITIES IN HELIUM NANODROPLETS by Yanfei Ren A Dissertation Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In partial Fulfillment of the Requirement for the Degree DOCTOR OF PHILOSOPHY (PHYSICS) December 2007 Copyright 2007 Yanfei Ren |
Comments
Post a Comment for Page 1
