Electronic Structure of theUCenter. II. Force-Constant Changes and Local Modes

The force constants and local-mode frequencies of the $U$ center in KCl, KBr, and KI have been obtained from a detailed quantum-mechanical calculation. The model which has been used corresponds to the "static-well" approximation. It takes into account both the electronic structure of the first-nearest-neighbor ions and the polarization of the ${\mathrm{H}}^{-}$ ion during the local-mode oscillations. The agreement of the calculated local-mode frequencies with the experimental values is greatly enhanced when the polarization of the ${\mathrm{H}}^{-}$ ion is included.