The probability of inner shell ionization of atoms by electron impact

Abstract

In the past few years a considerable amount of experimental work has been carried out on the investigation of the excitation of the optical energy levels of atoms by electron impact, and the probability of excitation of a given level has measured as a function of the energy of the incident electrons. However, little has so far been done on the analogous problem in the production of characteristic X radiation, viz., the investigation of the relation between the probability of ionization of an atom in an inner shell by electron impact and the energy of the impinging electrons. This problem has been discussed theoretically by Bethe using Born's approximate collision theory, which assumes that the impinging electron can be represented by a plane wave, and in which no account is taken of the distortion of this wave by the field of the atom with which it collides. The effect of this distortion will be negligible only when the energy of the incident electron is large compared with the ionization energy of The atomic shell with which it interacts. Under such conditions Bethe has found that f (V), the probability of inner shell ionization of an atom by impact with an electron of energy V e , can be expressed to a first approximation in the form f (V) = k /V log 4V/B nl V k , (1) in which V k e is the ionization energy of the shell being investigated, B nl is a parameter, constant for a given shell, and of the order of magnitude of unity and k = π e 3 n 2 Z nl b nl /R h Z eff 2 . (2)