SOFT-ELECTRON EMISSION PEAK IN ION-HELIUM COLLISIONS

We study the angular distribution of electrons, emitted in ion-helium collisions, with low velocities relative to the target. We use an expa nsion of the ionization doubly differential cross section, in Legendre polynomials, with electron velocity-dependent weight coefficients tha t define the angular shape of the distribution. We determine these coe fficients from available experimental data for H+, He-2+, C6+ , and O8 + projectiles at 1- and 1.84-MeV impact energies, and we evaluate them by using the first Born and the continuum-distorted-wave-eikonal-init ial-state (CDW-EIS) approximations. The experimental results and CDW-E IS calculations confirm the existence of forward-backward asymmetry in the soft-electron emission peak. We discuss the dependence of theoret ical and experimental parameters on the impact energy and the charge o f the projectile.