Optical absorption spectra of X-ray irradiated alkaline earth fluoride crystals doped with divalent rare-earth ions studied by thermal bleaching

The absorption spectra of X-ray irradiated alkaline earth fluoride (CaF2, SrF2, BaF2) crystals doped with Sm, Ho, Er, Tm ions are investigated by thermal bleaching of the sample in the 300-700 K temperature range. X-ray irradiation creates absorption bands of inter-configuration 4f(n)-4f(n-1).5d(1) transitions of divalent impurity ions and F-3(-) and (F-2(-))(ii) hole centers. The thermal oxidation of the divalent RE ions have been observed. Two possible mechanism of thermal oxidation of divalent lanthanide ions in alkaline-earth fluorides are discussed.