Resonance structure in the electron-impact excitation of Ca sup + below the 5 s threshold

Detailed calculations of energies and widths of Ca autoionizing states and the related resonance structure in electron-impact excitation of the 4{ital s}-4{ital p} transition in Ca{sup +} are performed using the diagonalization method. This method is divided into two steps: a three-state (4{ital s},3{ital d},4{ital p}) close-coupling calculation of cross sections and extensive configuration-interaction calculations of wave functions of autoionizing states. Both steps take into account the correlations related to one- and two-body core-polarization potentials. The results of calculations of energies, oscillator strengths, and autoionizing widths of doubly excited states of Ca are in better agreement with the experimental data than any other calculation. Rich resonance structure is predicted by theory at incident electron energies below the 5{ital s} threshold. The present results support the general pattern of the measurements of Frontov (Opt. Spectrosc. 9, 460 (1985)), although there are some discrepancies in the details, particularly the existence of the high and broad resonance at that threshold. A detailed comparison with the previous resonance calculations by Mitroy {ital et} {ital al}. (Phys. Rev. A 38, 3339 (1988)) is carried out.