Infrared spectroscopic study of the dressed rotations ofCN−isotopes in alkali halide crystals

Abstract

High-resolution infrared absorption spectroscopy has been used to study the roto-vibrational spectra of cyanide ions doped into KCl for the two isotopes ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}_{}^{\mathrm{-}}{}_{}{}^{}$ and ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}_{}^{\mathrm{-}}{}_{}{}^{}$ in the temperature range 1.7–300 K. At the lowest temperature studied, new structure has been resolved in the spectra for low dopant concentrations (0.01 mol %). This structure is observed to merge into a single peak when strains are introduced into the crystal, confirming the association of these lines with tunneling by isolated cyanide ions. The structure is only partially explained by the Devonshire potential. Above 100 K, the isotopic variation in the experimental data is quantified in terms of a single global parameter, an integrated ratio of the widths of the entire roto-vibrational manifolds for the two isotopes, at each temperature. The values of this parameter turn out to be much smaller than what is predicted by a straightforward model of a cyanide dipole freely rotating in a crystalline host, but are consistent with a dressed-rotor model.