Libration of the Hydroxyl Ion in Alkali Halide Crystals

Abstract

This paper presents experimental data for a direct absorption band near 300 ${\mathrm{cm}}^{-1\mathrm{}}$ due to the hydroxyl ion in several alkali halides. This excitation is also seen as a combination band 300 ${\mathrm{cm}}^{-1\mathrm{}}$ above the 3600-${\mathrm{cm}}^{-1\mathrm{}}$ O—H stretching band. The lines are broad, even at helium temperatures, and seem to be Lorentzian. Dichroism induced by a static electric field indicates that the transition dipole moment is perpendicular to the O—H axis. The Devonshire model is unable to explain the O${\mathrm{H}}^{-}$-O${\mathrm{D}}^{-}$ isotope shifts, and predicts more than the one strong band which is seen. A simple torsional harmonic-oscillator model can explain relatively well the isotope shifts and the oscillator strengths for both the direct librator and the librational sideband. Better agreement can be obtained by assuming some anharmonicity in the angular potential energy and/or assuming a slight displacement of the axis of rotational oscillation from the c.m. The temperature dependence of the linewidths may indicate a coupling between the torsional librational oscillator and the "30-${\mathrm{cm}}^{-1\mathrm{}}$" or "non-Devonshire" levels studied earlier.