Impurity-Induced Second-Harmonic Infrared Absorption and Raman Scattering byU-Center Localized Modes in Alkali-Halide Crystals

Abstract

The addition of cation impurities to alkali-halide crystals containing $U$ centers substitutionally in the anion sublattice removes the center of inversion from a certain fraction of the $U$-center sites. The potential energy of such $U$ centers consequently acquires nonzero cubic anharmonic terms which are absent in the absence of the additive cation impurities. The cubic anharmonic terms allow electric dipole transitions to occur between the ground state and the second excited state of the localized modes associated with the $U$ center. These transitions can be observed as second harmonics of the fundamental lattice absorption by the $U$-center localized vibration modes. The strength of such impurity-induced second-harmonic absorption has been calculated for the system NaCl: K. In addition, the removal of the center of inversion from certain $U$-center sites by the addition of cation impurities has the consequence that the localized vibration modes associated with $U$ centers at such sites, which in the absence of the additive cation impurities do not give rise to a first-order Raman scattering of light, therefore become Raman-active in first order. The Raman scattering efficiency for the first-order scattering by $U$-center localized modes induced by additive cation impurities in a typical alkali-halide crystal is calculated. In the particular case of NaCl: ${\mathrm{H}}^{\ensuremath{-}}$ containing 5% KCl, the scattering efficiency is estimated to be 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}12}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ sec ${\mathrm{sr}}^{\ensuremath{-}1}$.