A semiempirical calculation of the axial shift of the self-trapped exciton in KCl

A method of calculating electric-field gradients is given for the off-centred self-trapped exciton in KCl, first observed by Block, Wasiela and Merle d'Aubigne (1978). The method is based on the model for defect formation in alkali halides described previously and closely follows the treatment by Gourary and Adrian (1960) for F-centres. Using this expressions obtained for the electric-field gradients and the observed values of the quadrupole interaction parameters for the two central chlorine ions, the magnitude of the axial displacement is calculated to be 0.22 Bohr radii.