Differential cross sections for ionization of atomic hydrogen and helium by electron and proton impact

Abstract
The classical impulse approximation to the differential cross sections for electron and proton ionization of atomic hydrogen and helium are calculated using accurate electron velocity distributions for the atoms. The results are compared with quantal Born calculations. It is shown that the classical impulse and quantal Born differential cross sections agree over a wide range of energies of both the incident and ejected particles. If the energies of the incident and ejected particles are both large it is proved that the classical impulse, Thomson and Born approximations with all give the same differential cross sections for ionization of atomic hydrogen. For proton ionization of helium the classical impulse gives differential cross sections which agree quite well with measurements.

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