Johnstone, Wyndham Michael; (1990) Electron scattering from polyatomic molecules. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The design and use of a high resolution electron spectrometer to measure absolute elastic collision cross sections for electron scattering from molecular targets is described. The spectrometer was constructed around two 180° hemispherical energy selectors which were employed to define the energy spread of the incident electron beam and to analyse the energy of the scattered electrons. Electrons emitted from a thoriated tungsten hairpin filament were transported to and from the selectors using several two and three element cylinder lenses. The total resolution of the spectrometer was variable between 28meV for an incident beam current of 2nA and 100meV for currents in excess of 60nA. The range of incident energies and scattering angles at which electron scattering could be measured with the spectrometer was 2-100eV and 10°-120°, respectively. Three polyatomic molecules have been studied, SF6, H2O and N2O and absolute differential cross sections for electron scattering from these molecules have been measured for incident energies between 5eV and 80eV. By extrapolation of the differential cross sections to 0° and 180°, integral and momentum transfer cross section were also obtained. A two stage process was employed in the determination of the absolute differential cross sections. The relative differential cross section (normalised to 90°) was first measured for scattering angles between 10° and 120° using the subtraction technique. In this method problems arising from the overlap geometry of the gas and electron beam plus background effects were accounted for. The relative differential cross sections were then normalised to an absolute scale using the absolute differential cross section at 90° obtained using the relative flow technique. The error associated with the absolute differential cross sections obtained using this approach were ≈ 14%.

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