Extensions of the Wannier theory for near-threshold excitation and ionisation of atoms by electron impact

Abstract

The classical cross sections for excitation and ionisation processes in which the total angular momentum is zero and the magnitude of the total energy E (measured with respect to the ionisation energy) is small are investigated. The cross sections have the form mod E mod 0.127f(E1/ mod E mod ) when the charge of the residual ion is unity, where E1 is the asymptotic energy of the more energetic of the two final electrons and the cross sections are differential in E1. The forms of f(E1/ mod E mod ) for E<0 and E>0 have been established by accurate trajectory integrations. The results are found to be consistent with those of the various experimental studies of near-threshold excitation of high-n states and near-threshold ionisation. The distribution functions of the mutual angle theta 12 in the ionisation process and of the individual electron angular momenta l in the excitation process have been investigated, and their energy dependences and forms have been found.