Electron Impact Ionisation of Ground-State and Laser-Excited Atoms

Abstract

(e, 2e) experiments test our understanding of collision dynamics in the electron impact ionisation process. These experiments use coincidence techniques to ascertain a triple differential cross section (TDCS) for the ionisation process, by measuring the angular correlation between the electron scattered from the target and the bound electron that is produced during ionisation. Almost all measurements conducted so far have been from atoms and molecules in their ground state. The original aim of this work was to measure (e, 2e) processes from laser-excited and aligned atoms, since this provides additional information on the collision from these targets. By aligning the atoms initially using a laser, the spherical symmetry of the atom is reduced, so that a quadruple differential cross section must be measured which depends on the scattering geometry as well as the alignment angle of the atom. Results from this type of experiment are expected to aid in our understanding of collisions involving molecules, since these targets also have reduced symmetry compared to atoms. An open-source laser-atom interaction simulator derived from quantum electrodynamics (LASED) has been built to allow the simulation of the dynamics of any atom excited by continuous wave, narrow bandwidth laser radiation. Simulation results for the $4^1S_0$ to $4^1P_1$ laser-excitation of Ca, $3^1D_2$ to $10^1P_1$ laser-excitation of He after electron-impact excitation from the $^1S_0$ ground state, and the laser-excitation of the Cs $D_2$ line are hence presented in this thesis as examples of the application of LASED. Two different (e, 2e) spectrometers were used to make measurements presented in this thesis from ground state noble gas targets. A modular, customisable, and low-cost experimental control system for these spectrometers using LabVIEW and Arduino interfaces has been built and is described in this thesis. This control system, combined with a new LabVIEW laser locking program will allow laser-aligned (e, 2e) experiments to be performed without an operator present in the future. A laser interlock system required to satisfy new health and safety regulations is also detailed. Measurements of the TDCS of Xe as the scattering geometry was varied from a coplanar geometry to the perpendicular plane are detailed in this thesis. These measurements were taken from 60 eV to 100 eV above the ionisation potential (IP). Data for the TDCS of Ar in the perpendicular plane from 5 eV to 200 eV above the IP are also presented in this thesis. These data confirm the results of previous measurements at Manchester and reveal a trend in the flattening of the TDCS as the incident energy increases. A new technique for mapping the measurements of the relative TDCS of Xe to an absolute scale using a He-Xe mixture is presented. The new Ar data also reveals a deep minimum in the TDCS at 50 eV above the IP, which may be due to a nearby quantum vortex. Preliminary experiments to measure laser-aligned (e, 2e) from strontium are also outlined, so that future research using this target has guidance as to where to search for signal.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	George King (Supervisor) & Andrew Murray (Supervisor)