Recent experiments on the roles of projectile electrons in ion-atom collisions

By measuring recoil-ion-electron, projectile-electron, and triple coincidences, multiple ionization differential cross sections and probabilities for impact of H, He, H+, He and He++ colliding with He, Ne, Ar and Kr have been obtained. Here we concentrate on differential measurements of multiple ionization of heavy targets produced by impact of neutral and charged H and He projectiles. In the case of H+ impact, previous experiments have shown a large enhancement of electron emission associated with the ratio of Ar2+/Ar+, with a maximum at an electron velocity close to the projectile velocity for electron emission angles around 90 degrees. Both, double ionization (DI) and transfer ionization (TI) are responsible for the enhancement, which was also observed for Ne and Kr targets. New results demonstrate that, for 100 keV H++Ar collisions, the contribution to the enhancement due to DI is larger than TI. Neutral beams produce no effect of this sort. A simultaneous projectile and target ionization contribution is discussed.