Evidence for electron-electron interaction in projectileK-shell ionization

Cross sections for projectile K-shell ionization were measured for 0.75–3.5-MeV/N ${\mathrm{C}}^{5+}$ and ${\mathrm{O}}^{7+}$ projectiles in collisions with ${\mathrm{H}}_2$ and He targets. The experimental results agree with plane-wave Born-approximation calculations which take into account the interaction between projectile and target electrons. We demonstrate that for energies where the target electrons have sufficient kinetic energy in the projectile frame to ionize the projectile electron, the electron-electron interaction can lead to a significant increase in the total ionization cross section.