Postcollision effects in target ionization by ion impact at large momentum transfer

Abstract

We have measured and calculated fully differential cross sections for target ionization in 16-MeV O${}^{7+}$+He and 24-MeV O${}^{8+}$+Li collisions. As in previous studies, in the case of the He target we observe a pronounced forward shift in the angular distribution of the electrons relative to the direction of the momentum transfer q at small $q$ ($q$ 1 a.u.). An unexpected result is that we also find a strong forward shift at large $q$ ($q$ > 2 a.u.), while at intermediate $q$ this shift becomes very weak or even turns into a backward shift. For the Li target, in contrast, the forward shift monotonically increases with increasing $q$. These observations are qualitatively reproduced by our calculations. The comparison to theory suggests that at large $q$ the forward shift is due to the postcollision interaction between the outgoing projectile and the ejected electron, but at small $q$ it is mostly due to an interplay between the projectile-target core interaction and the electron-target core interaction.