Indirect processes in the electron-impact ionization of Li-like ions

Detailed measurements of electron-impact ionization cross sections have been made in the vicinity of the excitation-autoionization thresholds of Li-like B2+, C3+, N4+, O5+ and F6+ ions. With an energy spread of 0.4 to 1% of the electron energy and statistical uncertainties as low as 0.1% we could clearly resolve thresholds for excitation of individual terms in 1s 2s 2l configurations. Numerous resonance features were found which are due to dielectronic capture of the incident electron with subsequent two-electron emission. In particular, dielectronic capture processes involving Δn=2 excitations of a 1s electron provide the dominant resonance contributions to the measured cross sections. Rydberg series 1s 2snln'l' of resonances with n=3 (Δn=2) and n=4 (Δn=3) are resolved up to principle quantum numbers n' equal to 6 or 7. (orig.)