Calculations of the contributions of excitation-autoionisation to the electron impact ionisation of Ca+and Ba+in the distorted-wave approximation

The contributions of the excitation-autoionisation transitions 3p 6 4s to 3p 5 3d4s in Ca + and 5p 6 6s to 5p 5 5d6s in Ba + to the total electron impact ionisation cross sections of these ions have been calculated in the distorted-wave approximation. Partial cross sections originating from transitions to three of the nine terms in the configuration 3p 5 3d4s of Ca + are compared with the recent R-matrix close-coupling calculations for these transitions by Burke et al. (1983) in order to determine the effect of continuum coupling between autoionising channels. Both the R-matrix and distorted-wave calculations predict a series of large peaks near threshold in Ca + that are not observed in the measurements of Peart and Dolder (1968). Total cross sections with estimates for the contributions of direct ionisation are calculated and compared with the ionisation measurements of Peart and Dolder for Ca + and of Peart et al. (1973) for Ba + . Both LS term dependence in the nd radial wavefunctions and unitarisation are found to have a pronounced effect on these cross sections. The implications of the discrepancies between theory and experiment are discussed.