Application of the binary-encounter theory to proton impact double ionisation of atoms

Proton impact double ionisation of He has been investigated in the double-encounter model of Gryzinski (1965). The binary-encounter expressions developed by Vriens along with an accurate quantum-mechanical velocity distribution for the bound electrons in both encounters have been used to obtain double-ionisation cross sections. These results have been compared with the experimental observations and earlier calculations by Gryzinski's method. The variation of sigma + / sigma ++ (single-ionisation cross section/double-ionisation cross section) with incident proton energy is also found to be in qualitative agreement with the experimentally observed variation.