Dissertation Defenses

Doctoral Dissertation Announcement

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Candidate: Susanta Das

Degree of: Doctor of Philosophy

Department: Physics

Title: Transmission and Guiding of Fast Electrons through Insulating Nanocapillaries and Comparison with Ion Guiding

Committee:
Dr. John A. Tanis, Chair
Dr. Pnina Ari-Gur
Dr. Asghar Kayani
Dr. Nikolaus Stolterfoht

Date: Monday, December 8, 2008 4:00 p.m. - 6:00 p.m.
2202 Everett Tower

Abstract:
Transmission and guiding of fast electrons (500 and 1000 eV) through an insulating polyethylene terephthalate nanocapillary foil has been investigated and compared with results for slow highly-charged ions. As for slow ions, guiding is attributed to charge-up of the inner walls near the capillary entrance, which, after a characteristic time, electrostatically deflects the traversing ions causing them to be guided through the sample along the capillary axis. The measurements were performed at WMU. Electron guiding is found to decrease faster with both energy and foil tilt angle than for ions. Ions lose negligible energy during the course of guiding, and, furthermore, do not appreciably change charge state. However, the spectra of transmitted electrons through the foil exhibit significant energy losses that increase with energy and tilt angle. The energy losses suggest that electrons undergo multiple elastic and inelastic scattering within the capillaries before being transmitted or lost inside the foil, a phenomenon not observed for slow ion guiding. Despite the considerable energy losses, it has been found that the inelastically as well as the elastically scattered electrons are guided through the capillaries. It is suggested that the various inelastic processes associated with electron guiding are due to insufficient deposition of electron charge on the inner walls of the capillaries to prevent a major fraction of the traversing electrons from interacting strongly with the capillary walls. These inelastic processes are also the likely cause of lower transmission intensities for electrons compared to ions. The present results indicate that electron and ion guiding are qualitatively and quantitatively different processes. When the measurements were later repeated at 500 eV using the same sample to confirm the guiding and the associated energy losses, a large reduction in the magnitude of the transmitted intensities and a greater fall-off with increasing tilt angle were observed, suggesting deterioration of the sample since the earlier work. Possible reasons for the deterioration include changes in the properties of the polymer. Studies of the same sample with fast ions show that the transmitted ions undergo significant energy losses and charge exchange within the sample, supporting deterioration of the sample.