Mapstone, Isabella M.; (1990) An experimental study of elastic and inelastic electron scattering from molecules. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The high resolution electron spectrometer designed and constructed for the present work is described. Throughout the design particular attention was paid to the window and pupil positions, beam and pencil angles and the effect of these parameters on the overall resolution of the spectrometer. Two sets of 180° hemispherical analysers, fitted with Jost type correction plates to reduce the fringing fields, were used to define the energy spread in the incident electron beam and analyse the energy of the scattered electrons. Instrumental energy resolutions of around 50 to 60meV for primary beam currents of 2 to 3nA across the interaction region were achieved. Details of the interface built for a BBC microcomputer and the programs written to control it are given. This interface was used to collect the experimental data by generating a ramp voltage under the control of the BBC micro. For each of the 1024 steps in the ramp voltage it was possible to count pulses simultaneously from two independent electron detectors, these counts being stored in two 1024 element arrays. On completion of the required number of scans the energy loss spectrum could be displayed. The number of scans, step time, and initial and final steps in the ramp are all input parameters in the data acquisition programs. The spectrometer and interface were used to carry out a series of measurements of the elastic differential cross sections of methane, ethene and ethane and of the excitation of vibrational levels in the electron impact energy range 3eV to 15eV for scattering angles of 30 to 140°. Equations are derived to enable the calculation of elastic and inelastic differential cross sections from these measurements and the details of the programs written to fit the energy loss spectra are given. Methane, ethane find to a lesser extent ethene are all present in the atmospheres of Jupiter, Saturn and its satellite Titan. These molecules are also of interest due to their different symmetries, and as a testing ground for collision theory. Recent developments in computational techniques allow accurate calculations of the cross sections of large molecules to be made, consequently good experimental data is required for critical comparison.

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