Elastic scattering of quasi-free electrons in strongly asymmetric collisions

A systematic study of the doubly differential cross sections for the electron emission occurring in fast strongly asymmetric collisions is presented. Protons and atomic hydrogen (0.5 MeV) were collided with the heavy targets krypton and xenon under single collision conditions. The ejected electrons were observed at laboratory angles in the range 0'<8,5180"and with energies E,from 25 to 1400eV. In addition to the familiar strong ejection in the forward direction (0,%0') arising from capture and loss to continuum states there is pronounced emission in the backward hemisphere for the case of atomic hydrogen projectiles. This.is accompanied by large variations in the angular dependent singly differential cross section and in the energy and width of the electron loss peak as measured at particular angles These Rudtuations in the ability of a heavy target to ionize the structured projectile are a manifestation of a RamsauerTownsend efect and can be understood within the framework of quasi-free electron scattering by the target potential. We give a relativistic quantum mechanical model based on the electron impact approximation which shows good agreement with experiment for these strongly asymmetric collision systems.