Fast oscillating structures in electron spectra followingHeq++Hecollisions (q=1,2) at low projectile energies

Abstract

Distributions of ejected electrons following collisions of slow He{sup +} and He{sup 2+} ions and a He target were measured for projectile energies of 20 and 40 keV, respectively. The electrons were detected at angles of 30 deg. and 90 deg. with respect to the incident beam direction. Superimposed on a continuous background originating from target ionization, small amplitude, high-frequency oscillations are revealed. The frequency of these oscillations is found to be nearly independent of the projectile charge and observation angle. In view of recent experiments and calculations, the origin of such oscillations is discussed. Processes such as autoionization following the production of highly excited states, Fermi-shuttle ionization, or coherent electron emission caused by interference between the target centered and projectile centered amplitudes, are considered.