IONIZATION SURFACES FOR SMALL MOLECULES

An nb initio molecular orbital method is used to calculate total ioniz ation cross sections for small molecules as a function of the projecti le electron-molecule orientation. Calculations over many angles are us ed to construct three-dimensional ionization surfaces for molecules, s uch that, if an electron penetrates the surface, ionization follows. T otal 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 calc ulated from the binary-encounter-Bethe method and the additivity metho d of Deutsch and Mark. Good agreement between the orientation-averaged cross section, experiment, and other theories, and between the calcul ated and experimental steric factor for the total ionization of CH3Cl supports the concept of orientation dependence for the electron impact ionization probability.