Unexpected Charge-State Dependence of $K$ X Rays Produced in Gaseous Targets by ${\mathrm{S}}^{+q}$ and ${\mathrm{Ti}}^{+q}$ Ions

$K$ x rays produced in the bombardment of gas jet targets of helium, oxygen, and argon by sulfer and titanium beams have been measured for projectile charge states of +6 to +16 and +8 to +19, respectively. Unexpectedly, the charge-state dependence is similar for nearly symmetric systems and for very asymmetric systems. The results suggest that direct Coulomb excitation into empty bound states could be a common $K$-shell-vacancy production mechanism.