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 expansion of the ionization doubly differential cross section, in Legendre polynomials, with electron velocity-dependent weight coefficients that define the angular shape of the distribution. We determine these coefficients from available experimental data for H+, He2+, 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-initial-state (CDW-EIS) approximations. The experimental results and CDW-EIS calculations confirm the existence of forward-backward asymmetry in the soft-electron emission peak. We discuss the dependence of theoretical and experimental parameters on the impact energy and the charge of the projectile