Ionization phenomena in high-energy atomic collisions

Abstract

The statistical theory of ionization produced by atomic collisions is here improved and extended, in line with the concept that the observed ionization is due to quasimolecular excitation followed by auto-ionization transitions in each of the colliding atoms after the collision is completed. The assumptions of the statistical theory are recast in the form of assumptions regarding the relative sizes of matrix elements in the quantum theory of transition rates, generalized to treat multiple ionization processes occuring in a single transition. The relation of the statistical theory with the Fano-Lichten model is discussed. In addition to calculating the probabilities that the colliding atoms will be found in each of the possible states of ionization, an expression for the energy spectrum of the ejected electrons is also obtained. It is also demonstrated that multiple excitation of the outer shell, as occurs in atom-atom collisions, plays a crucial role in determining the probability that a given state of ionization will result from the de-excitation process. The theory permits the comparison of the matrix elements for multiple ionization from those auto-ionizing states in which the outer shell is highly excited (as arises in atomic collisions) with the matrix elements for multiple ionization from those auto-ionization states in which the outer shell is unexcited (as arises from photo-ionization of an inner shell).