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

Abstract

We present experimental measurements and theoretical calculations for the electron-impact single ionization cross section of ${\mathrm{Sc}}^{+}$ ions covering an energy range from threshold to $1000\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. 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 $\ensuremath{\sim}40%$. This finding is related to the existence of strong $3p\ensuremath{\rightarrow}3d$ excitation channels in ${\mathrm{Sc}}^{+}(3{p}^{6}3d4s)$. The shape of the related cross-section feature is reminiscent of the very strong $4d\ensuremath{\rightarrow}4f$ excitation, found in the ionization of xenon and its neighboring elements.