Raman and Vibronic Spectra of the Nitrite Ion in Alkali Halides

Raman and vibronic absorption spectra associated with N$\mathrm{O}_2^{}{}_{}{}^{-}$ substitutional impurity in KCl, KBr, KI, and NaBr at 6 °K are reported. The equilibrium orientation of the nitrite ion at low temperatures is determined from the stress-induced dichroism or splitting of the vibronic lines. Rotational fine structure on these lines is analyzed in terms of hindered rotation of the nitrite ion. Structure at higher frequencies in the vibronic and Raman spectra is assigned to perturbed lattice modes.