Raman scattering by the hydroxyl ion in sodium chloride

We have measured Raman scattering from OH− ions in several NaCl:OH− crystals at low frequency shifts and in the O–H stretching region. The best results were obtained with an impurity concentration of 170 ppm. At low temperatures and frequencies we have observed an A1g-Eg ``tunneling'' transition at 3.2 cm−1, plus other Eg and T2g lines, but no A1g lines. In the stretching region there is a set of resolved lines that show a complicated temperature dependence merging into broad T2g and E bands at temperatures higher than 28 K. To develop a consistent explanation for these Raman data and for previous infrared data, we adopted a model due to Keller and Kneubühl. We assigned many of the transitions to the motion of the displaced center of mass of the OH− ion, with the O–H bond direction following adiabatically. The problem then reduces to that considered by Devonshire. The fitted values for the Devonshire potential parameter K were 16 and 7.5 cm−1 for ground and excited stretching states, and the moments of inertia were 13 and 16 amu · Å2, respectively. The experimental Raman cross sections were compared with those calculated using wavefunctions valid in 2 limits; tight-well tunneling and free rotor. The experimental situation represents an intermediate case. The comparisons yielded values for the anisotropic part of the polarizability tensor, its derivative with respect to the stretching coordinate, and the derivative of the trace of the polarizability tensor. The broad, depolarized higher temperature bands could be understood semiquantitatively by assuming that the nearly free rotational states are strongly affected by ``collisions'' with phonons, collisions that are not frequent enough to cause true rotational diffusion. The A1g stretching line showed collisional narrowing. We applied a theory that assumed the unnarrowed line shape would be caused by a difference between the rotation constants for ground and excited stretching states. The theory yielded estimates of the rotational correlation time that imply a mean angle of turn between collisions of about 1 rad.