Explanation of the T- and X-line components of the emission of ns2 impurity ions in alkali halides by lattice relaxation studies

Accurate lattice relaxation studies were performed on the impurity centres Ga, In and Tl in FCC KCl, when these ions are excited from the ground ns2 state to the triplet nsnp state. Using HF-LCAO as the basic model for the electronic structure a satisfying explanation was found for the T-lines of the three impurity ions. Particularly this is hard for thallium where there is a strong competition between lattice relaxation and the quenching of the spin-orbit interaction. An unexpected explanation was found for the X-type emission line of which the centre geometry is unknown. It is very likely emitted from a centre where the impurity ion is displaced over a long distance in one of the (1 1 1) directions.