Raman Scattering from Tunnelling and Hindered Rotational States of CN − Molecules in KCl

The low lying tunnelling and hindered rotational states of dilute (≈ 10 −4 ) systems of substitutional CN − ions in KCl are studied by Raman techniques. The tunnelling splitting of the ground state could be directly observed as a resolved side‐band structure of the CN − stretching vibration. The observed splitting is in quantitative agreement with the previously derived value for the tunnelling splitting, while the appearance of this splitting only in the T 2g scattering geometry confirms definitely the 〈111〉 dipole model. Hindered rotational structure (in the 10 to 30 cm −1 range), which is partially resolved, is detected, both in the low energy and stretching‐side‐band spectrum in E g and T 2g symmetry. The observed Raman data can be fitted to the Raman positions and intensities calculated by Beyeler, using the V 4 + V 6 Devonshire model parameters derived from high resolution IR work.