Many-body Green's function theory of electron impact ionization

Abstract

Using the methods of many-body Green's function formalism, several different formally exact (and therefore impractical for computation) expressions are derived for the S matrix for electron impact ionization of atoms (molecules). The expressions differ in the lowest order where computation is possible. The physical models implied by the various low-order expressions are discussed. Computationally simple formulae which are physically reasonable result from the analysis. Consideration of the physics suggests that different formulae will apply in different energy regimes. The suggested key to understanding the physics is a 'two-time' model which considers relative passage and excitation (ionization) times. A direction for new work on the general electron scattering and ionization problem based on two-time ideas is discussed.