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 domin ate the differential cross sections for low energy target electron emi ssion induced by fast neutral projectile impact on any target. Measure ments of the 15-degrees electron emission were made in order to invest igate this prediction. For low impact energies, a constant ratio betwe en the hydrogen atom and proton impact cross sections was found for em itted 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 electr on velocity decreased. This is interpreted as a signature of projectil e electron-target electron interactions becoming dominant for distant collisions between neutral particles.