Graduation Semester and Year

2023

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics and Applied Physics

Department

Physics

First Advisor

Alex H Weiss

Abstract

This dissertation presents measurements and theoretical modeling demonstrating the capability of Doppler Broadened annihilation gamma Spectroscopy (DBS) to provide element-specific information from the topmost atomic layer of surfaces that are either clean or covered with adsorbates or thin films. Our measurements show that the energy spectra of Doppler-shifted annihilation gamma photons emitted following the annihilation of positrons from the topmost atomic layers of clean gold (Au) and copper (Cu) differ significantly. With the aid of the positron annihilation-induced Auger electron spectroscopy (PAES) performed simultaneously with DBS, we show that measurable differences between the Doppler broadened gamma spectra from Au and Cu surfaces in the high energy region of the gamma spectra can be used for the quantification of surface chemical composition. Modeling the measured Doppler spectra from clean Au and Cu surfaces using gamma spectra obtained from ab initio calculations after considering the detector energy resolution and surface positronium formation pointed to an increase in the relative contribution of gamma from positron annihilations with valence shell electrons. The fit result also suggests that the surface-trapped positrons predominantly annihilated with the delocalized valence shell (s and p) electrons that extended into the vacuum than the highly localized d electrons. Simultaneous DBS and PAES measurements from adsorbate (sulfur, oxygen, carbon) or thin film (selenium, graphene) covered Cu surface showed that it is possible to distinguish and quantify the surface adsorbate and thin-film composition just based on DBS. DBS measurements of annihilation gamma line shapes for each elemental surface present a promising avenue for developing a characterization tool that can be used to probe external and internal surfaces that are inaccessible by conventional surface science techniques. Additionally, by detailed analysis of the timing spectra obtained in the UTA TOF-PAES spectrometers and Monte Carlo Simulation, we were able to explore the impact of contributions from the self-annihilation of surface formed Ps on the ability of DBS to estimate the elemental composition of the topmost atomic layer and it includes discussion about the potential of combining DBS with machine learning algorithms for accurate and efficient surface characterization.

Keywords

Positron, Positronium, Doppler broadening gamma spectroscopy, Surface characterization, Monte Carlo simulation, Machine learning classification

Disciplines

Physical Sciences and Mathematics | Physics

Comments

Degree granted by The University of Texas at Arlington

Available for download on Saturday, February 01, 2025

Included in

Physics Commons

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