Infrared absorption and rotational dynamics ofOH−andOD−defects in cesium halides

The rotational degrees of freedom of substitutional ${\mathrm{OH}}^{\mathrm{-}}$ and ${\mathrm{OD}}^{\mathrm{-}}$ defects in CsCl, CsBr, and CsI have been studied between 2 and 300 K using infrared absorption in the range of the 2.7 and 3.7-μm stretching absorption. In CsCl the first direct evidence for quasi-free-rotational behavior of ${\mathrm{OH}}^{\mathrm{-}}$ in alkali-metal halides is found in the form of a vibration-rotation spectrum above 45 K, corresponding to a reorientational barrier of ∼30 ${\mathrm{cm}}^{\mathrm{-}1}$. The small rotational constant of B=1 ${\mathrm{cm}}^{\mathrm{-}1}$ without ${\mathrm{OH}}^{\mathrm{-}}$-${\mathrm{OD}}^{\mathrm{-}}$ isotope shift, derived from the temperature dependence of the rotational P and R branches, indicates a considerably increased moment of inertia for ${\mathrm{OH}}^{\mathrm{-}}$ and possibly off-center motion of the molecule around the H ion. High-frequency librational sidebands observed below and above 45 K and their isotope shifts indicate radial and angular modes superimposed on the rotational motion and can be explained with a simple double-librator model. In CsBr and CsI doped with ${\mathrm{OH}}^{\mathrm{-}}$ and ${\mathrm{OD}}^{\mathrm{-}}$, high-frequency sidebands and isotope shifts expected from a harmonic-librator model for center-of-mass libration in high barrier potentials are found. The observed second harmonic of the stretching absorption in CsCl and CsBr allows determination of the mechanical and electrical anharmonicity parameters.