Role of ionization-excitation processes in the cross section for direct ionization of heavy atomic ions by electron impact

Abstract

The contribution to the ionization cross section of ionization-excitation processes by electron impact is usually negligibly small for low- and medium-$Z$ elements. We demonstrate here, however, that for heavy atomic ions with the outermost shell being $nd$ $(n=4,5)$ the ionization-excitation processes play an evident role in the ionization cross section. For the $4{s}^{2}4{p}^{6}4{d}^{10}$ ground level of ${\mathrm{Gd}}^{18+}$, the ionization-excitation cross section due to the excitation of levels in the $4{s}^{2}4{p}^{6}4{d}^{8}4f$ configuration is comparable to the direct $4p$ and $4s$ ionization cross sections of ${(4{s}^{2}4{p}^{5}4{d}^{10})}_{1/2}$ and ${(4s4{p}^{6}4{d}^{10})}_{1/2}$. The total ionization cross section will be underestimated by 15% without including the contribution from ionization-excitation processes. This is a general conclusion for heavy atomic ions, which is verified by taking Pd-like ions of ${\mathrm{Sn}}^{4+},{\mathrm{Ba}}^{10+},{\mathrm{Nd}}^{14+},{\mathrm{Tb}}^{19+},{\mathrm{Yb}}^{24+}$, and ${\mathrm{W}}^{28+}$ as examples. The role of ionization-excitation processes can be understood from the overlapping of the wave functions between the $4d$ and $4f$ orbitals.