ELECTRON-ELECTRON INTERACTIONS IN FAST NEUTRAL-NEUTRAL COLLISIONS

Differential electron emission is studied for 50-500 keV H+ and H atom impact on helium. Using the first Born formulation, it is shown that projectile electron-target electron interactions are expected to dominate the differential cross sections for low energy target electron emission induced by fast neutral projectile impact on any target. Measurements of the 15-degrees electron emission were made in order to investigate this prediction. For low impact energies, a constant ratio between the hydrogen atom and proton impact cross sections was found for emitted electron velocities less than half the projectile velocity, V(p). But as the collision energy increased, for electron velocities less than 0.25V(p), the cross section ratio increased as the emitted electron velocity decreased. This is interpreted as a signature of projectile electron-target electron interactions becoming dominant for distant collisions between neutral particles.