Electronic Processes in the Off-Centre Instability for Cu⁺ Impurity Ions in Alkali Chlorides

Abstract

The off-centre instability for Cu⁺ impurity ions in alkali chlorides was studied using a model that the ground state with A_1g symmetry and a localized excited state with T_1u symmetry mix with each other through the odd-mode electron-lattice interaction. In addition, an absorption lineshape due to a transition from the ground state to an even-parity state was calculated. The instability and the optical properties of the Cu⁺ ion revealed through earlier empirical works are explained by this model. Among possible candidates for the T_1u state, a charge-transfer excitation state between the Cu⁺ and the neighbouring Cl⁻ is concluded to play an essential role in the optical properties and the off-centre instability.