Effect of uniaxial stress on luminescence of undoped and thallium-doped KI and RbI crystals

The effect of uniaxial stress applied at various temperatures along the (100) crystal axis on the self-trapped exciton (STE) and impurity-induced luminescence has been studied for pure and Tl+, Na+-containing KI and RbI crystals under excitation in the exciton and band-to-band absorption region as well as under x-ray excitation. An increase of the intrinsic/extrinsic emission intensity ratios has been observed and explained by the stress-induced (i) enhancement of the self-trapping efficiency of electronic excitations and the decrease of their migration length and (ii) suppression of the nonradiative decay of excitons into stable radiation defects. A strong influence of the uniaxial stress on the structure of the STE adiabatic potential energy surface has been detected in RbI which is evident in the stress-induced increase of the Ex/π and, particularly, of the σ/Ex emission intensity ratios. This effect has been connected with the increase in the energy barriers between various STE configurations in the compressed crystal lattice. The dependence of the on-centre STE emission intensity in alkali halides on the distance between the nearest lattice anions and on their size has been discussed. (author)