Single ionization is a three-body momentum exchange process. One can approximate it by splitting it in three separate two-body momentum exchange processes between projectile-electron, projectile-ion and electron-ion.
Although all three pair-wise interactions are always present, there are paradigmatic cases where one of the three dominates.

The figure shows the momenta of electron and ion and the momentum loss of the projectile in the plane defined by the incoming projectile (horizontal from left to right) and the scatterd projectile. At slow collisions clearly the main momentum exchange is between projectile and target, while the electron is stranded with very little momentum. At fast p-He collisions in contrast significant momentum is exchanged in particular between projectile and electron (see and in very fast highly charged ion collisions the main momentum exchange is between electron and its parent ion (for more details see Moshammerxxx, these experiments are performed at GSI by R.Moshammer(Robert.Moshammer), J. Ullrich (Joachim.Ullrich) and collegues).