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 1 s2 s2 l 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 1 s electron provide the dominant resonance contributions to the measured cross sections. Rydberg series 1 s2 snln′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. © 1990 Springer-Verlag.