EPR Studies of Photochromic CaF2

The stable photochromic center Ca${\mathrm{F}}_2$ was found to be a trivalent rare-earth ion (either La, Ce, Gd, Tb, Lu, or Y) with a fluorine-vacancy immediately next to it, plus two electrons making a neutral complex. This stable state can be ionized by irradiation with blue light ($\lambda \le 4000$ Å), leaving the trivalent rare-earth ion next to a perturbed $F$ center. The electron can be trapped by a rare-earth ion which occupies a cubic site forming a divalent rare-earth ion. Electron paramagnetic resonance (EPR) studies of all three types of centers are discussed.