Cross sections for inner-shell excitation of C3+, N4+and O5+ions

Cross sections calculated in a six-state close-coupling approximation are given for electron impact excitation of the 1s inner shell of the Li-like ions C 3+ , N 4+ and O 5+ for energies from near threshold to three times the excitation threshold. The 1s 2 2s to 1s2s2l excitation followed by autoionisation gives an abrupt and significant contribution to the total ionisation cross section, in agreement with recent measurements of Crandall et al. (1979). When combined with theoretical direct ionisation cross sections, the calculations agree with the observation that the relative contribution of the excitation-autoionisation process increases with increasing ionic charge for the three cases calculated for the Li isoelectronic sequence.