Alkali halide crystals containing impurity ions with the ns2 ground-state electronic configuration†

When impurity ions with the ns2 ground-state configuration are incorporated substitutionally in alkali halide crystals, they become phosphors. Because of the relatively simple electronic structure of these color centers it is possible to give a reasonably complete account of their properties including energy levels, absorption and MCD line-shapes, relaxed excited states, kinetics and polarization of the emission, and the role of various perturbations: electron-lattice interaction, spin-orbit coupling and magnetic fields. Although ab initio calculations are still lacking, it will be shown that our understanding of these systems is reasonably complete at the level of linear vibronic interactions and the harmonic approximation.