Charge transfer and F type centers in alkaline-earth fluorides activated by Cd$^+v_{a}$ or Zn$^+v_{a}$ ions

The photoluminescence spectra of impurity centers containing the anionic vacancy Cd$^+v_{a}$ or Zn$^+v_{a}$ in alkaline-earth fluorides are studied. A comparison with the luminescence spectra of photochromic centers (РC), which can be considered as perturbed F centers, shows that the luminescence of the Cd$^+v_{a}$ or Zn$^+v_{a}$ centers is similar to the luminescence of F centers and, therefore, is associated with charge transfer from an impurity ion to an anionic vacancy.