Electron-electron interaction in projectile electron loss

In ion-atom collisions where the projectile is ionized, target electrons act not only coherently by screening the target nucleus but they may also act incoherently by directly ejecting a projectile electron. This electron-electron interaction should be relatively most important for targets that have a low nuclear charge, since the cross section for a neutral target is roughly proportional to Zₜ²+Zₜ, where Zₜ² is the contribution due to the target nucleus and Zₜ comes from the target electrons. In order to investigate the electron-electron interaction, we have measured and calculated cross sections for Li²⁺, C⁵⁺, and O⁷⁺ on H₂ and He, Au⁵²⁺ on H₂, He, C, and N₂, Au⁷⁵⁺ on H₂ and N₂, U⁸⁶⁺ on H₂ and He, and U⁹⁰⁺ on H₂. The collision energies range from 0.75 to 405 MeV/nucleon. The calculations have been performed in the plane-wave Born approximation. We demonstrate that for energies where the target electrons have sufficient kinetic energy in the projectile frame to ionize the projectile, the electron-electron interaction can lead to a significant increase in the total electron-loss cross section.