Infrared Absorption of the Hydroxyl Ion in Alkali Halide Crystals

The O—H stretching band and its immediate sidebands have been studied for O${\mathrm{H}}^{-}$ impurities in NaCl, KCl, RbCl, KBr, KI, and NaBr from 1.4 to 300°K. Sideband splittings range from 12 ${\mathrm{cm}}^{-1\mathrm{}}$ in NaCl:O${\mathrm{H}}^{-}$ to 37 ${\mathrm{cm}}^{-1\mathrm{}}$ in KBr:O${\mathrm{H}}^{-}$, and correlate with dips in the low-temperature thermal conductivity and with far-infrared absorption bands. Preliminary results on the electric-field-induced dichroism in KCl:O${\mathrm{H}}^{-}$ and RbCl:O${\mathrm{H}}^{-}$ give a dipole transition moment perpendicular to the O—H axis for the main sideband. There is evidence that several sidebands in NaCl:O${\mathrm{H}}^{-}$ are themselves combination bands with one or more 2-3-${\mathrm{cm}}^{-1\mathrm{}}$ tunneling levels. The first overtone yields values of ${\omega }_e=3790\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and ${\omega }_e{x}_e=85.5\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ for O${\mathrm{H}}^{-}$ in KBr. These values explain very well the O${\mathrm{H}}^{-}$/O${\mathrm{D}}^{-}$ isotope shift of the main stretching band in KBr and KCl. The main-band oscillator strengths are small, on the order of 5×${10}^{-3\mathrm{}}$ for most crystals, but this value is consistent with a theoretical calculation of the dipole moment by Cade. The main bands broaden rapidly with increasing temperature, possibly indicating a prominent lifetime-limiting role played by the sideband levels. The sideband levels themselves are tentatively suggested to be due to a center-of-mass resonance in KBr:O${\mathrm{H}}^{-}$ and to a librational resonance or fast tunneling level in KCl:O${\mathrm{H}}^{-}$.