Optical Studies of a Photochromic Color Center in Rare-Earth-Doped CaF2

An impurity-associated color center is found in Ca${\mathrm{F}}_{2}$ doped with La, Ce, Gd, Tb, Lu, or Y. Photo-ionization by uv light leads to a reversible color change, i.e., the material is photo-chromic. This paper presents the results of optical experiments including (a) magnetic circular dichroism and (b) linear dichroism produced by exposure to linearly polarized light. Identification of the center's absorption spectrum, both before and after photo-ionization, is made. The simplest consistent model is two electrons trapped by a trivalent-rare-earth fluorine-vacancy nearest-neighbor complex.