Abstract
An ab initio molecular orbital method is used to calculate total ionization cross sections for small molecules as a function of the projectile electron-molecule orientation. Calculations over many angles are used to construct three-dimensional ionization surfaces for molecules, such that, if an electron penetrates the surface, ionization follows. Total ionization cross sections deduced by averaging over the positive and negative Cartesian coordinates and from the volume enclosed by the ionization surface are compared to experimental values and those calculated from the binary-encounter-Bethe method and the additivity method of Deutsch and Mark. Good agreement between the orientation-averaged cross section, experiment, and other theories, and between the calculated and experimental steric factor for the total ionization of CH3Cl supports the concept of orientation dependence for the electron impact ionization probability.
Published Version
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