Abstract
Using a combined time-of-flight and coincidence technique the authors measure the total cross sections for simultaneous ionisation of both collision partners in 50-1200 keV on He and Xe collisions. For H in He the fraction of the total projectile ionisation cross section corresponding to the simultaneous target ionisation increases monotonously from a few per cent to about 50% in the present energy range. Contrary to this the same fraction for H in Xe shows a maximum of about 50% close to 200 keV followed by a steep decrease. In He above 200 keV the energy variation of the measured cross sections for simultaneous ionisation agrees well with calculations in the first Born approximation, but there is a small deviation in absolute magnitude. The qualitative features of the cross sections for simultaneous ionisation measured in Xe agree with recent predictions based on the free-collision model. Total ionisation cross sections for ground-state He atoms colliding with ground-state H atoms which have not been measured previously are also presented.

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