Defect pairs ofFcenters and substitutionalH−ions (Ucenters) in alkali halides. Resonance Raman studies

Defect pairs of $F$ and $U$ centers ["$F_H\left({\mathrm{H}}^{-}\right)$ centers"] were observed for the first time, created by laser-light induced aggregation of $F$ centers in NaCl, KCl, and RbCl crystals doped with high concentrations of substitutional ${\mathrm{H}}^{-}$ defects ($U$ centers). The formation of these pairs was indentified by the appearance of three sharp ${\mathrm{H}}^{-}$ local-mode lines in the Raman spectrum of $F$ centers (due to the replacement of one of the 12 ${\mathrm{Cl}}^{-}$ neighbors by a small-mass ${\mathrm{H}}^{-}$ ion). Raman studies of these local mode lines observed in different scattering geometries ($A_{1g}, E_g, T_{2g}$) and in first and second order, reveal their symmetry and identify them as the three orthogonal linear vibrations of the ${\mathrm{H}}^{-}$ defect in a $〈110〉$ pair configuration with the $F$ center. A simple model, based on the Van der Waal interaction between the vibrating $H^{-}$ dipole and the induced electronic dipole at the $F$ center, explains well the observed local-model shifts. A speculative discussion is given on the absorption, emission, and photochemical properties of this pair formed from the two simplest prototypes of an electronic and local-model defect