Local modes of ${\mathrm{H}}^{-}$${\mathrm{H}}^{-}$, ${\mathrm{D}}^{-}$${\mathrm{D}}^{-}$, and ${\mathrm{H}}^{-}$${\mathrm{D}}^{-}$ pairs in KBr, KI, NaCl, and RbCl

The main coupling between localized ${\mathrm{H}}^{-}$ or ${\mathrm{D}}^{-}$ harmonic oscillators in KCl, KBr, KI, NaCl, and RbCl is confirmed to be the dipole-dipole interaction. Several configurations for the ${\mathrm{H}}^{-}$ ${\mathrm{H}}^{-}$ or ${\mathrm{D}}^{-}$ ${\mathrm{D}}^{-}$ pairs were detected and their mode frequencies were found to be in accordance with the dipole-dipole interaction. The absorption line of one of the transverse modes of the $〈110〉$ neighboring ${\mathrm{H}}^{-}$ ${\mathrm{H}}^{-}$ pairs in KBr was found to be composed of four peaks. This structure is shown to be related to critical points in the phonon dispersion curves of the pure KBr crystal. The approach to the pair-mode linewidth proposed in a previous paper is shown to be valid in its general features.