Irradiation-Induced Color Centers in Magnesium Fluoride

Color centers, produced by 50-kV x rays, have been studied in the rutile-structured Mg${\mathrm{F}}_2$, largely at room temperature. An initial, slightly anisotropic, absorption band near 260 nm is tentatively identified as due to $F$ centers. Subsequent optical bleaching results in the formation of a strong band at 370 nm and several weak bands, including one at 320 nm. Both of these appear only for $\mathbf{E}\perp c$ and are believed to be due to $M$ centers. This choice of models is suggested, in part, by the observation of typical trapped-electron ESR spectra in irradiated samples. Bleaching studies show that the 320-nm band arises from centers lying along the $〈110〉$ directions, while the 370-nm band seems to be cylindrically symmetric about the $c$ axis. Furthermore, the 320-nm band is reversibly convertible into the 370-nm band. Selection rules have been derived for ${\mathrm{}\left(\mathrm{ls}\right)\mathrm{}}^2\to \mathrm{}\left(\mathrm{ls}2p\right)\mathrm{}$ transitions in each of the four possible $M$-center configurations permitted in the rutile structure. It is found that of the two types having F-F bonds in the (001) plane, one identifies rather well with the 370-nm band and the other with the 320-nm band.