Ionization Produced by Atomic Collisions at kev Energies

Abstract

A phenomenological theory is developed which accounts for the ionization produced by single collisions between heavy atoms or ions at kiloelectron volt energies. The collision ionization is regarded as a two-step process. First, as the two electron distributions sweep through each other, a certain amount of energy is transferred by a friction-like mechanism from the kinetic energy of translation of the atoms to their internal degrees of freedom. Second, this transferred energy, which is analogous to heat energy, is statistically distributed among the electrons. The probability that any given number of electrons acquire more than the ionization energy is then computed by a straightforward statistical analysis. The probabilities that the collision products are in the various states of ionization are thereby calculated as functions of the collision parameters. This ionization mechanism is analogous to the evaporation of molecules from a heated liquid.The theory is compared with experiment for the case in which singly charged argon ions are scattered by neutral argon atoms at energies of 25, 50, and 100 kev. At each bombarding energy the probabilities of finding the detected atom in any of the charge states, from zero to seven times ionized, as functions of the angle of scattering are predicted with reasonable accuracy with only two adjustable parameters.