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High resolution electron impact studies Thomas, Gary E.

Abstract

A wide variety of inelastic scattering and electron attachment phenomena have been studied by directing an electron beam into a gas filled chamber and by observing the results of the interaction. The use of a "monoenergetic" electron beam has permitted the relative cross sections for such processes to be examined in some detail. Three instruments were used to make the measurements, and these instruments were designed and constructed during the course of the work to be described. All three employed a 127° electrostatic electron velocity selector to filter the electron beam. An electron energy analyzer permitted the observation of inelastically scattered electrons, and a monopole or a quadrupole mass filter permitted the measurement of the relative amounts of positive or negative ionization generated by the electron beam in its interaction with the gas. The measurement of the energy dependence of the cross sections was the prime objective of the work, and much care was taken in the design of the instruments to ensure that the cross section curves were not distorted by spurious effects. The forward inelastic scattering spectra of helium and ethylene were recorded using an electron beam having an incident energy of 50 eV . The ionization efficiency curves of singly and doubly charged helium were obtained, and the results are compared with several theoretically derived expressions for electron impact threshold laws. The positive ionization efficiency curves for nitrogen and oxygen were also obtained. An examination of the dissociative capture, non-dissociative capture and ion pair processes has been made by determining the energy dependence of the cross sections for the production of the parent and/or fragment negative ions from nine molecules. Finally, the generation of zero energy electrons from inelastic scattering events has been studied by using sulfur hexafluoride as an electron scavenger in a mixture with the sample gas. The threshold electron impact spectra were obtained for six atomic and molecular species. In all cases, such measurements were carried out below and above the ionization potential of the sample.

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