Charge-state dependence of characteristicKx-ray emission in near-symmetric and asymmetric collisions

The projectile charge-state ($q$) dependence of characteristic $K$ x-ray production has been studied for a variety of collision systems, ${\mathrm{Cu}}^{q+}\to \mathrm{H}\mathrm{e},\mathrm{N}\mathrm{e},\mathrm{K}\mathrm{r},\mathrm{Z}\mathrm{n}$, and ${\mathrm{Br}}^{q+}\to \mathrm{Kr}$, at 0.87 and 1.40 MeV/amu. All targets were of gas or vapor type to provide single-collision conditions. An increase in the $K$-excitation cross sections occurs for projectiles having performed $2p$ vacancies, the absolute cross section increase per incoming $2p$ vacancy being orders of magnitude larger for near-symmetric than for asymmetric collision systems. We identify the difference in magnitude with the predominance of a different excitation mechanism in the two cases, contrary to the view put forward recently by Tserruya et al.