Graduate Studies

 

First Advisor

Martin Centurion

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Physics and Astronomy

Date of this Version

7-2024

Document Type

Dissertation

Citation

A dissertation presented to the faculty of the Graduate College of the University of Nebraska in partial fulfillment of requirements for the degree of Doctor of Philosophy

Major: Physics and Astronomy

Under the supervision of Professor Martin Centurion

Lincoln, Nebraska, July 2024

Comments

Copyright 2024, Mahmudal Hasan. Used by permission

Abstract

Dissociative electron attachment (DEA) is a resonant process in which a molecule captures a low-energy electron, forming a transient negative ion (TNI). Subsequently, this unstable TNI fragments into a stable anion and one or more neutral fragments. DEA is crucial in various phenomena, ranging from atmospheric and radiation chemistry to processes occurring in plasmas, and is particularly significant in the context of radiation-induced damage to biological molecules. This study uses different experimental methods to better understand the fragmentation in molecular anions forming through dissociative photoexcitation and electron attachment. We have developed an experimental apparatus for dissociative photoexcitation studies. This apparatus generates, manipulates, and analyzes ion-molecule clusters and their fragmentation pattern. We also utilized an apparatus located at Lawrence Berkeley National Laboratory to image the three-dimensional momentum distribution of negative ions produced through DEA to molecular targets. In this dissertation, first, we describe the design of a reflectron time-of-flight (TOF) mass spectrometer, which was implemented in our experimental apparatus - designed to investigate the dissociation dynamics of photoexcited ion-molecule clusters by mass-resolving and detecting fragment anions and neutrals. Then, our experimental work demonstrates previously untested aspects of dissociative photoexcitation in molecular anions with this apparatus. We investigate the DEA process in which multiple TNI states are accessed by absorbing varying numbers of photons. This multiphoton absorption method allowed us to investigate the excited states of molecular anions that may not be accessible through a single-photon absorption or with an electron beam. The significance of our approach is that it does not rely on tunable electron or laser sources to access different TNI states. Finally, we present DEA studies with an external electron beam to investigate the formation of TNIs and fragmentation patterns in different organic molecules using an anion fragment momentum imaging apparatus. We have investigated the TNI formation following low-energy electron attachment to acetic acid and its partially- and fully-deuterated isotopologues in the dissociation channels leading to H- and D- formation. Our results confirmed three previously known resonance positions and identified a fourth resonance that had not been reported earlier. We also examined the anion fragment yields from DEA to 1-M-5-Nitroimidazole (1M5NI) at different electron energy. This data can be used to simulate electron-induced radiation damage in biologically relevant media containing 1M5NI as a potential radiosensitizer.

Advisor: Martin Centurion

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