PHOTO-REVERSIBLE CHARGE TRANSFER IN RARE-EARTH-DOPED-CaF2

Photoreversible absorption changes have been found in additively colored La-, Ce-, Gd-, or Tb- doped CaF2. Results are shown for one of these, Ce:CaF2, in which both the trivalent and divalent rare-earth absorptions are identifiable. These results suggest a charge transfer between the rare earth and a defect center produced by additive coloration.