ELASTIC-SCATTERING OF QUASI-FREE ELECTRONS IN STRONGLY ASYMMETRIC COLLISIONS

A systematic study of the doubly differential cross sections for the e lectron emission occurring in fast strongly asymmetric collisions is p resented. Protons and atomic hydrogen (0.5 MeV) were collided with the heavy targets krypton and xenon under single collision conditions. Th e ejected electrons were observed at laboratory angles in the range 0- degrees less-than-or-equal-to theta(f) less-than-or-equal-to 180-degre es and with energies E(f) from 25 to 1400 eV. In addition to the famil iar strong ejection in the forward direction (theta(f) almost-equal-to 0-degrees) 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 fluctuations in the ability of a heavy target to ionize the stru ctured projectile are a manifestation of a Ramsauer-Townsend effect an d can be understood within the framework of quasi-free electron scatte ring by the target potential. We give a relativistic quantum mechanica l model based on the electron impact approximation which shows good ag reement with experiment for these strongly asymmetric collision system s.