Microscopic response effects in collisions of antiprotons with helium atoms and lithium ions

We present total single- and double-ionization cross sections for the collision systems p(bar sign)+He and p(bar sign)+Li{sup +} in the 1-1000 keV impact energy range with emphasis on microscopic response effects during the collision. The calculations rely on the basis generator method. In both collision systems, the response of the effective electronic interaction to the time-dependent density reduces the ionization cross section at low impact energies significantly. It is shown, in comparison with other theories, that this reduction is a consequence of time-dependent screening rather than due to dynamical correlation effects.