Ionization in fast atom-atom collisions: The influence and scaling behavior of electron-electron and electron-nucleus interactions

We report cross sections for ionization of He coincident with electron loss from He, Li, C, O, and Ne pro jectiles. For He, Li, C, and O pro jectiles, the cross sections were measured directly, while the Ne cross sections were obtained by transforming results for He pro jectiles colliding with Ne. We find that, at energies of about 100-500 keV/u, neutral pro jectiles can ionize a He target almost as effectively as a charged pro jectile. The contribution to ionization due to electron-electron interactions is found to scale with the number of available pro jectile electrons. Comparing ionization by the bound electrons on projectiles to ionization by free electrons, we find the cross sections for ionization by bound electrons are systematically smaller than those for free electrons.