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 ${\mathit{Z}}_{\mathit{t}}^2$+${\mathit{Z}}_{\mathit{t}}$, where ${\mathit{Z}}_{\mathit{t}}^2$ is the contribution due to the target nucleus and ${\mathit{Z}}_{\mathit{t}}$ comes from the target electrons. In order to investigate the electron-electron interaction, we have measured and calculated cross sections for ${\mathrm{Li}}^{2+}$, ${\mathrm{C}}^{5+}$, and ${\mathrm{O}}^{7+}$ on ${\mathrm{H}}_2$ and He, ${\mathrm{Au}}^{52+}$ on ${\mathrm{H}}_2$, He, C, and ${\mathrm{N}}_2$, ${\mathrm{Au}}^{75+}$ on ${\mathrm{H}}_2$ and ${\mathrm{N}}_2$, ${\mathrm{U}}^{86+}$ on ${\mathrm{H}}_2$ and He, and ${\mathrm{U}}^{90+}$ on ${\mathrm{H}}_2$. 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.