Color Centers in Alkaline Earth Fluorides

Single crystals of Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, and Ba${\mathrm{F}}_2$ have been colored by x rays (150 kv) and high-energy electrons (2.5 Mev). Five color-center bands in Ca${\mathrm{F}}_2$ and Sr${\mathrm{F}}_2$ and nine in Ba${\mathrm{F}}_2$ have been observed in the spectral region between 0.175 and 0.8 μ. At room temperature the bands in Ca${\mathrm{F}}_2$ are well resolved, while those in Sr${\mathrm{F}}_2$ and Ba${\mathrm{F}}_2$ overlap. At -190° and -255°C the bands sharpen but generally do not shift, except the 580-mμ band in Ca${\mathrm{F}}_2$ which shifts toward longer wavelengths. Photochemical bleaching of the strongest band shows a certain influence on the others. Three bands could be correlated to the Mollwo relation with the exponent 3.85. A possible correlation of the absorption bands to certain models is discussed.