Strong contributions of indirect processes to the electron-impact ionization cross section of Sc⁺ ions

We present experimental measurements and theoretical calculations for the electron-impact single ionization cross section of Sc⁺ ions covering an energy range from threshold to 1000 eV. An electron-ion crossed-beams setup was employed for the measurements of absolute cross sections as well as for a high-resolution energy scan to uncover fine details in the energy dependence of the cross section. Direct ionization is described by configuration-averaged distorted-wave theory and indirect ionization by R-matrix theory. Indirect processes contribute to the total ionization cross section by up to ∼40%. This finding is related to the existence of strong 3p→3d excitation channels in Sc⁺(3p⁶3d4s). The shape of the related cross-section feature is reminiscent of the very strong 4d→4f excitation, found in the ionization of xenon and its neighboring elements.