High-energy optical-absorption bands of transition-metal ions in fluoride host crystals

Absorption bands in the region from 40 000 to 80 000 ${\mathrm{cm}}^{-1\mathrm{}}$ have been found for the divalent ions of V, Cr, Mn, Fe, Co, and Ni in the host crystals KMg${\mathrm{F}}_3$, Ca${\mathrm{F}}_2$, and Mg${\mathrm{F}}_2$. Their spectral positions are near those of the free-ion $3d\to 4s$ transitions when corrected for crystal-field shifts. The $f$ numbers are near ${10}^{-3\mathrm{}}$ and most of the bands have a temperature-dependent absorption strength consistent with a vibronic absorption mechanism. It is concluded that these are well described as $3d\to 4s$ transitions.