Electron-impact ionization cross sections for He-like ${\mathrm{Li}}^{+}$ ions have been measured with extreme precision (0.01% statistics). Thus, two new mechanisms in the ionization of positive ions both involving two-electron excitations could be quantitatively investigated. We see (1) direct excitation of both K electrons into the L shell with subsequent autoionization and (2) resonant capture of the incident electron by the ion with correlated KK-LL excitation of both core electrons leading to a triply excited Li atom which then decays by the emission of two correlated electrons. Strong interference of process (2) with direct ionization is observed.

• Received 28 March 1989

DOI:https://doi.org/10.1103/PhysRevLett.63.758