Single and double ionization of helium in relatively small-impact-parameter collisions with 0.5–2.5-MeV/u ${\mathrm{O}}^{6,7+}$ ions is studied by associating target ionization with single-electron loss from the projectile. Double ionization is considerably enhanced (4–6 times) and found to deviate substantially from the energy and charge-state behavior observed for double ionization without accompanying electron loss from the projectile. The results are considered in terms of one- and two-step processes of double ionization.

• Received 17 December 1990

DOI:https://doi.org/10.1103/PhysRevA.43.4723