Ultraviolet Absorption Spectra of Photochromic Centers in CaF2Crystals

Photochromic color centers are known to exist in additively colored Ca${\mathrm{F}}_2$ doped with La, Ce, Gd, Tb, Lu, or Y. It is also known that these centers consist of one or two electrons bound at the fluorine vacancy adjacent to the trivalent impurity cation. The ultraviolet (uv) absorption spectra of photochromic centers (PC) in Ca${\mathrm{F}}_2$ were measured from 35 000 to 80 000 ${\mathrm{cm}}^{-1\mathrm{}}$, the band edge of the host crystal, at both room and liquid-nitrogen temperatures. The spectra show two regions of absorption. The low-energy region, up to 57 000 ${\mathrm{cm}}^{-1\mathrm{}}$, has bands at 44 000 and 52 000 ${\mathrm{cm}}^{-1\mathrm{}}$, which are conjectured to be analogous to the $L$ bands of $F_A$ centers in alkali halides. The high-energy region, from 57 000 ${\mathrm{cm}}^{-1\mathrm{}}$ on, is characterized by a band at 62 000 ${\mathrm{cm}}^{-1\mathrm{}}$, which is interpreted as charge transfer from PC to nearby impurities. At the band edge, an extra absorption appears in the uv-switched Ca${\mathrm{F}}_2$: 0.1-at.%-Gd sample, and it is speculated that it is the $4f^7\to 4f^{6}5d$ transition of ${\mathrm{Gd}}^{3+}$. This interconfigurational transition is "red shifted" due to the screening effect of electrons, which have been released from the ionized PC and trapped in the vicinity of ${\mathrm{Gd}}^{3+}$.