Candidate: Osama Abu-Haija

Degree of: Doctor of Philosophy

Department: Physics

Title: Single-Electron Capture Processes in Slow Collisions of He^2+ With Molecular Targets

Committee: Dr. Emanuel Y. Kamber, Chair
Dr. Thomas Gorczyca
Dr. Nora Berrah
Dr. Sung Cheng

Date: Tuesday, May 10, 2005 3:00 p.m.-5:00 p.m.
Bradley Commons, 2202 Everett Tower

Abstract: Translational energy-gain spectroscopy (TES) has been used to investigate state-selective dissociative and non-dissociative single-electron capture processes in low-energy collisions of He 2+ ions with O 2 , H 2 O, CO 2 , N 2 , and NH 3 at impact energies between 100 eV and 1600 eV. The measured energy spectra for the He 2+ - O 2 and H 2 O collision systems show that the dominant exit channel is due to dissociative transfer ionization (DTI), single-electron capture accompanied by ionization of the molecular target-ion, at the lowest collision energies. As the impact energy is increased non-dissociative single-electron capture (SEC) into the He + (n=2) state is found to populate at collision energies in excess of 1000 eV. For N 2 and CO 2 targets, the dominant reaction is due to DTI. In the case of the He 2+ - NH 3 collisions, it is the SEC that is predominantly populated. The dominant reaction channels observed for collisions of He 2+ with N 2 , CO 2 , and NH 3 remain dominant over the entire impact energy region studied and at laboratory scattering angles between 0 ° and 8 ° . The energy-gain spectra are interpreted qualitatively in terms of the reaction windows, which are calculated using the Landau-Zener model and the extended version of the classical over-the-barrier model. The energy dependence of total cross sections for single-electron capture was also measured and found to be weakly dependent on the projectile energy. The present cross sections are also compared with the available data and are interpreted in terms of theoretical formulations based on Landua-Zener and Demkov models. These calculations show that the He + (n=2) formation (i.e., SEC) proceeds through a single-electron processes governed by electron-nucleus interactions. In contrast, the He + (n=1) formation (i.e., DTI) mechanisms involves an exothermic two-electron process driven by electron-electron interactions.

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