Lattice relaxation of luminescence centres of the X-line emission of ns2 impurity ions in alkali halides;: anomalous geometry of the impurity centre in the excited nsnp state

Clear evidence was found that in the active centre from which the X-line emission of ns(2) impurities in alkali halides originates the excited impurity ion is statically displaced over an anomalously large distance in a direction which depends on the lattice structure. For fcc this is one of the (111) directions and the Centre has trigonal symmetry, and for the bcc lattice it is one of the (001) directions and the symmetry is tetragonal. The displacement only occurs in the excited state and probably is a result of covalency effects. The study was performed using a readily available code for lattice relaxation around impurity ions. For the fcc lattice we have studied the relaxation of Gal, In+ and Tl+ in KCl and of In+ in Y-Br. For the bcc case we considered In+ in high-pressure KCl. A comparison with well known experimental data for the three impurity ions and the good agreement found for both the T- and X-line emission wavelengths supports our theoretical results.