Resonant dielectronic and direct excitation in crystal channels

We have observed dielectronic and direct excitation of H-like ${\mathrm{S}}^{15+}$ and ${\mathrm{Ca}}^{19+}$ and He-like ${\mathrm{Ti}}^{20+}$ ions in silicon channels caused by collision with weakly bound target electrons which behave as a free-electron gas. As in vacuo, relaxation of the doubly excited states can occur radiatively leading to ions of decreased charge, but in a crystal channel collisional effects can cause double ionization. The effects are seen in both the x-ray yields and charge-state fractions, and, in the case of ${\mathrm{Ti}}^{20+}$, in charge-state x-ray coincidences.