ELECTRON-EMISSION FROM BINARY COLLISIONS WITH Z(PROJECTILE)LESS-THAN-OR-EQUAL-TO-Z(TARGET)

We have measured doubly differential cross sections for electron emiss ion in ion-atom collisions. In one set of experiments, 0.5 MeV/u He2projectiles impinged on the target atoms He, Ne, Ar, Kr and Xe and mol ecules N-2 and O-2; in another set, H+ (energy ranging from 2.0 to 6.0 MeV) and 0.75 MeV/u Nq+ (q = 2-7) collided with Ar targets. In both c ases, emitted electrons were observed under 0 degrees. Major interest was directed upon the position of the Binary Encounter Peak, which is commonly assumed to follow a simple kinematic model. Surprisingly the peak is significantly shifted towards lower energies. For comparison w e calculated spectra with the Plane Wave Born Approximation and the Pl ane Wave Approximation. These do not show in general good agreement wi th our experimental data, presenting large discrepancies not only in a bsolute values but also in the behaviour with the target atomic number .