  We present a theoretical model for atomic hydrogen ionization by electron
impact in the instantaneous approximation and the more accurate
non-instantaneous approach using the methods of Quantum Electrodynamics, for
the binary coplanar and the coplanar asymmetric geometries. All electrons are
described by plane wave functions in the coplanar binary geometry but in the
asymmetric geometry the ejected electron is described by a Sommerfeld-Maue wave
function. It is shown that the two models give the same results in the non
relativistic limit for the binary coplanar geometry where the interactions can
be treated as instantaneous. However, this is no longer true for the
relativistic case where one has to take into account both the instantaneous
interaction and the radiation interaction. These results are obtained in the
first order of perturbation theory.

Atomic Physics