Ionization in fast-neutral-particle-atom collisions: H and He atoms impacting on He.

Experimental and theoretical data pertaining to the doubly differential electron emission occurring in collisions between structured projectiles and atoms are presented. How one or two loosely bound projectile electrons influence the ionization process was investigated for fast H- and He-atom impact on helium. Standard noncoincidence and ionized projectile--emitted-electron coincidence techniques were used to study the electron emission for 0.5- and 1-MeV/amu impact energies. The experimental data demonstrate the importance of events where both the projectile and the target are ionized in a single collision. From a comparison of theoretical calculations made using the first-order Born approximation and the experimental data, it is concluded that single ionization of the target and of the projectile are adequately described by such a model but that improved models are essential in order to account for the observed simultaneous projectile-target ionization events.