Sum rules and electron-electron interaction in two-center scattering

The role of target-electron--projectile-electron interactions in projectile-electron loss is examined in the framework of the plane-wave-Born-approximation. The summation over target-electron states, which typically uses the closure approximation, is refined by using a sum rule for stopping power due to Bethe. The resulting expression for the cross section is compared with other modifications of the closure approximation and with the experimental H⁰+H⁰ electron-loss section.