Electron Impact Ionization of Nitrogen and Methane

UoM administered thesis: Phd

  • Authors:
  • Ahmad Sakaamini

Abstract

This thesis presents the results for experimental (e,2e) studies on both nitrogen (N2) and methane (CH4) molecules. The data from the experiments were compared to theoretical calculations using various models and approximation methods; the molecular three-body distorted wave (M3DW) approximation, the distorted wave Born approximation (DWBA), the Ward Macek (WM) approximation, the orientation-averaged molecular orbital (OAMO) approximation, the proper averaging (PA) approach, and the generalized Sturmian functions (GSF) approach. The Manchester (e,2e) spectrometer is described in detail along with new experimental hardware and software used for experimental control, optimization, data acquisition, extraction, visualization, and analysis. Two in-depth (e,2e) studies were carried out for the nitrogen (N2) molecule. The first was carried out in both coplanar and non-coplanar geometries, for incident electron energies ~10 and ~20eV above the ionization potential of the 3sigma_g, 1pi_u and 2sigma_u states of N2. The results used outgoing electron energies with E1 = E2 = 4.6 eV +/- 0.5 eV, E1 = E2 = 9.7 eV +/- 0.5 eV, E1 = 4.6 eV +/- 0.5 eV and E2 = 14.5 eV +/- 0.5 eV. The experimental results show that the cross-sections are very sensitive to the states from which the ionization occurs, as well as the incident energy and post collisional interactions. These measurements were compared to calculations based on three models, all of which confirmed the importance of post collision interactions at these energies. The second (e,2e) study was a continuation of the first. It was carried out with incident electron energies ~20 and ~40 eV above the ionization potential for the 3sigma_g and 1pi_u states of N2, using equal energies for the outgoing electrons. The data were obtained with the incident electron beam in the scattering plane. This experiment produced six sets of measurements of the ionization triple differential cross-sections, with fixed angles of 45 degrees, 90 degrees and 125 degrees with respect to the incident electron beam for one of the outgoing electrons. A disagreement was found between theory and experiment which is thought to be due to limitations of the OAMO technique used to average over all molecular orientations. The final (e,2e) study detailed in the thesis was carried out for ionization of the highest occupied molecular orbital (HOMO) 1t2 state and the next highest occupied molecular orbital (NHOMO) 2a1 state of CH4 at an incident electron beam energy of 250eV, for ejected electron energies of 50eV and 30eV. Five fixed scattering angles (20 degrees, 22.5 degrees, 25 degrees, 27.5 degrees, and 30 degrees) were used in these coplanar asymmetric experiments. The measured triple differential cross-sections were compared to experimental results of the Afyon group in Turkey. Both experimental results were also compared to theoretical calculations using the M3DW and GSF models. The results showed very good agreement between both experiment and theory. The structure and peak locations in the results for 50 eV ejected electrons are in good agreement with the theoretical calculations. However, the agreement at 30eV was not as satisfactory. The M3DW calculations were slightly better in their predictions of the location and magnitude of the experimental peaks than the GSF model. A significant portion of this thesis details the modernization of the Manchester (e,2e) spectrometer that was carried out by the author, and which was then used to perform the experiments that are detailed here. New computer-controlled variable voltage supplies were designed, built, and integrated with the (e,2e) spectrometer, alongside the new (e,2e) LabVIEW computer application used for experimental control and data acquisition. The inner-workings of the (e,2e) spectrometer, the vacuum system, and the experimental hardware and software are all detailed in the thesis.

Details

Original languageEnglish
Awarding Institution
Supervisors/Advisors
Award date1 Aug 2020