Infrared Absorption of the Hydroxyl Ion in Alkali Halide Crystals

Abstract

The O---H stretching band and its immediate sidebands have been studied for O${\mathrm{H}}^{\ensuremath{-}}$ impurities in NaCl, KCl, RbCl, KBr, KI, and NaBr from 1.4 to 300\ifmmode^\circ\else\textdegree\fi{}K. Sideband splittings range from 12 ${\mathrm{cm}}^{\ensuremath{-}1}$ in NaCl:O${\mathrm{H}}^{\ensuremath{-}}$ to 37 ${\mathrm{cm}}^{\ensuremath{-}1}$ in KBr:O${\mathrm{H}}^{\ensuremath{-}}$, 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${\mathrm{H}}^{\ensuremath{-}}$ and RbCl:O${\mathrm{H}}^{\ensuremath{-}}$ give a dipole transition moment perpendicular to the O---H axis for the main sideband. There is evidence that several sidebands in NaCl:O${\mathrm{H}}^{\ensuremath{-}}$ are themselves combination bands with one or more 2-3-${\mathrm{cm}}^{\ensuremath{-}1}$ tunneling levels. The first overtone yields values of ${\ensuremath{\omega}}_{e}=3790$ ${\mathrm{cm}}^{\ensuremath{-}1}$ and ${\ensuremath{\omega}}_{e}{x}_{e}=85.5$ ${\mathrm{cm}}^{\ensuremath{-}1}$ for O${\mathrm{H}}^{\ensuremath{-}}$ in KBr. These values explain very well the O${\mathrm{H}}^{\ensuremath{-}}$/O${\mathrm{D}}^{\ensuremath{-}}$ isotope shift of the main stretching band in KBr and KCl. The main-band oscillator strengths are small, on the order of 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ 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${\mathrm{H}}^{\ensuremath{-}}$ and to a librational resonance or fast tunneling level in KCl:O${\mathrm{H}}^{\ensuremath{-}}$.