Candidate: Ximao Feng

Degree of: Doctor of Philosophy

Department: Physics

Title: Photoexcitation and Photoionization of Ar Atoms and HBr Molecules

Committee: Dr. Nora Berrah, Chair
Dr. John Tanis
Dr. Thomas Gorczyca
Dr. Emanuel Kamber
Dr. William Liou

Date: Thursday, October 27, 2005 4:00 p.m.- 6:00 p.m.
2202 Everett Tower

Abstract: Abstract: Interaction between synchrotron radiation with atoms and molecules is one of the most important methods to study the structure and dynamics of atoms and molecules. This Theses concentrates on the study of the interaction between soft x-ray photons with Ar atoms and HBr molecules with time-of-flight (TOF) electron spectrometers and two-dimensional photoelectron spectroscopy (2DPES). Both of the experiments were performed on the Atomic, Molecular and Optical Physics undulator beam at the Advanced Light Source at Lawrence Berkeley National Laboratory.

The argon study focused on photoelectron recapture when the 2p electrons are ionized just above the thresholds of the two components. From our 2DPES maps, we obtained a semi-empirical recapture curve at the threshold as a function of photon energy. The curve is derived by extracting all the reemission photoelectrons from a specific recaptured intermediate excited state and subtracting the contributions from the 2p -1 nl resonances and the recapture at (over) the threshold. The experimentally obtained curve is in good agreement with both our semi-classical calculation result and the calculation performed with quantum-mechanics by Tulkki et al [Phys. Rev. A 41, 181 (1990)].

The HBr molecule study aimed mainly at measuring the atomic decay channel from the resonances. At these resonances, the repulsive characteristics of the antibonding orbital cause the molecules to dissociate, resulting in both atomic Auger decay (deexcitation happening after complete dissociation) and molecular Auger decay (deexcitation occurring before the dissociation is completed). We separate the two resonances for the first time and find that the peak positions are at 70.89 and 71.92 eV, respectively. Some atomic lines are separated for the first time and some are assigned based on their kinetic energy positions. We find that the profiles of the atomic Auger lines on the photon energy scale are not symmetric, reflecting the nonlinearity of the potential energy curve of the intermediate s* state within the Franck-Condon region and the dissociation rate dependence on photon energy. The angular distribution of the atomic Auger decay lines at the two resonances have been analyzed for the first time. The atomic line pair, produced from one of the two resonances and decaying to the same final Br + state, have approximately the same angular distribution parameters, s * . Comparison shows that the angular distributions of some of the atomic lines in HBr are apparently different from those of the corresponding lines in HCl. The intrinsic anisotropy parameters, a 2 , of the atomic lines, derived from the b values, are determined and are found similar to those of the equivalent M 4,5 N 2,3 N 2,3 , and M 4,5 N 1 N 2,3 , and M 4,5 N 1 N 1 Auger lines in krypton. The alignment parameters A 20 for the two resonances are found to be -0.64(5) ( 2 D 5/2 ) and -0.60(5) ( 2 D 3/2 ), respectively, which are in good agreement with the theoretical value of -0.74 predicted for the 2 D 5/2 resonance.

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