U-Center-Induced Raman Scattering in KBr and KI

Abstract

Raman scattering from the second harmonic state of the high-frequency $U$-center localized mode has been measured in KBr: ${\mathrm{H}}^{-}$, KBr: ${\mathrm{D}}^{-}$, KI: ${\mathrm{H}}^{-}$, and KI: ${\mathrm{D}}^{-}$ at 10 K. Lines of $A_{1g}$, $E_g$ and $T_{2g}$ symmetry are observed. The peak positions and symmetries of these lines are in agreement with the static-lattice model of the local mode. The anharmonic coupling coefficients needed to calculate the localized oscillator energy levels have been determined from the experimental data and used to describe the short-range repulsive potential in the neighborhood of the $U$ center. The importance of polarization effects in the interaction of the impurity with its surroundings is demonstrated. A weak peak of $A_{1g}$ symmetry has been observed in KI: ${\mathrm{H}}^{-}$ at about 95 ${\mathrm{cm}}^{-1\mathrm{}}$. It is attributed to a gap mode previously seen in infrared sideband measurements. Absolute Raman-scattering efficiencies and cross sections have been determined for all the impurity-induced modes in KI: ${\mathrm{H}}^{-}$, and the polarizability derivatives associated with the gap mode have been computed.