Electron-impact ionization in the iron isonuclear sequence

Excitation-autoionization contributions to the electron-impact ionization of Fe⁵⁺, Fe⁶⁺, Fe⁹⁺, Fe¹¹⁺, and Fe¹³⁺ are calculated in the distorted-wave approximation. Calculations for Fe⁵⁺ which include only excitations from the 3p⁶3d³ ground configuration agree well with the experimental crossed beams measurements of Gregory et al. [preceding paper, Phys. Rev. A 34, 3657 (1986)]. For Fe⁶⁺ the agreement between theory and experiment is also good. Calculations for Fe⁹⁺ are in reasonable agreement with experiment when only excitations from extremely metastable states of the 3p⁴3d excited configuration are included. Theoretical predictions for Fe¹¹⁺ and Fe¹³⁺ are based on ground-configuration excitations and detailed branching-ratio calculations. Excitation-auto- ionization contributions in Fe¹³⁺ are found to enhance the total ionization cross section by a factor of 2 near threshold.