Abstract
For electron impact ionization of hydrogen the S matrix is determined by calculating Feynman’s path integral semiclassically. The total ionization probability is in excellent agreement with the experiment. At a critical excess energy of 3.3 eV the differential cross section for the energy sharing between the continuum electrons undergoes a qualitative change which limits the range of the threshold behavior. The Wannier threshold law is confirmed, but only in the limt of vanishing excess energy.

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