Polarized Local-Mode Absorption of Aligned ${\mathrm{H}}^{-}$${\mathrm{H}}^{-}$, ${\mathrm{D}}^{-}$${\mathrm{D}}^{-}$, and ${\mathrm{H}}^{-}$${\mathrm{D}}^{-}$ Pairs in Alkali Halides

Aligned $U$-center pairs (containing ${\mathrm{H}}^{-}$ and ${\mathrm{D}}^{-}$ ions) were produced by a reaction of interstitial ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, and HD molecules with optically aligned $M$ centers. The study of their local-mode spectra revealed several new bands in the infrared. From the polarization of these local modes in aligned systems, the observed bands can be clearly assigned to the different optically active vibrations of the $〈110〉$-oriented pair centers.