Measured cross sections for ionisation of C³⁺, N⁴⁺and O⁵⁺ions with contribution due to excitation-autoionisation

Measured cross sections for electron impact ionisation of Li-like ions of C 3+ , N 4+ and O 5+ are reported for energies from threshold to 1500 eV. For electron energies from near threshold through the first peak in the cross sections, the measured values are in reasonable agreement with Coulomb-Born calculations. However, in each case an abrupt increase in the measured ionisation cross section is observed at the electron energy associated with excitation of an inner-shell electron, a process which has not been included in the theories. The 1s 2 2s to 1s2s2l excitation followed by autoionisation is shown to give an abrupt and significant contribution to the total ionisation cross section. Further, the relative contribution of the excitation-autoionisation process increases with increasing ionic (or nuclear) charge for the three cases tested along the Li isoelectronic sequence.