Rotational Raman effect: Molecular impurities in alkali halides

Raman scattering of light from representative alkali-halide crystals containing CN-, NO2-, OH-, and OD- impurities is reported and analyzed. The observed spectra have a low-frequency range in which the scattered light is usually shifted from the incident light by less than 300-400 cm-1, and a high-frequency range in which the shifts are typically 1000-2000 cm-1. Although the low-frequency region does not readily lend itself to quantitative analysis, it is clear that its main features can be interpreted in terms of a mixture of second-order scattering from the pure host, impurity-induced first-order scattering that results from perturbing the pure host, and scattering from the rotational degrees of freedom of the molecular impurity. The high-frequency region, on the other hand, consists of spectra whose frequencies are characteristic of the internal normal coordinates of the molecule. A very narrow totally polarized line with depolarized sideband structure is generally observed. The sharp central component is at the frequency of an internal molecular normal coordinate and, typically, has a linewidth of 1 cm-1. It is not significantly affected by the type of host or changes in temperature. It is found that the sideband structure gives a measure of the molecular rotational dynamics. Depending on host and impurity, the observed characteristic behavior varies from nearly free rotation to heavily trapped librational motion. The techniques employed here, both theoretical and experimental, demonstrate and define the usefulness of the Raman effect in studying systems of an analogous nature.