Effect of uniaxial stress on luminescence of X- and VUV-irradiated NaCl and NaBr crystals

The effect of uniaxial stress applied at 80 K along the < 110 > crystal axi s on the self-trapped exciton (STE) and impurity-induced luminescence has b een studied for NaCl and NaBr crystals under X-ray and VUV-light excitation . It has been shown that the intensities of both the pi and the sigma STE l uminescence bands increased in X-rayed NaCl more than 10 times under the ap plied uniaxial stress, whereas only a small stress-induced increase of the intrinsic luminescence has been observed in NaBr crystal. TSL measurements have shown that the creation of fundamental radiation defects (F-H pairs) i s strongly suppressed in stressed NaCl. It is concluded that the influence of the uniaxial stress is less pronounced in alkali halides with the on-cen tre final configuration of the STE (NaBr) whereas the stress-induced effect s may be very large in alkali halides in which the relaxation of the electr onic excitations occurs by means of the off-centre relaxation (NaCl, KI and RbI). In the latter case, a strong stress-induced effect is mainly connect ed with an increase in the energy barrier between the minima of two differe nt STE configurations, a radiative off-centre one and the one forming the r adiation defects (F-H pairs). (C) 2001 Elsevier Science Ltd. All rights res erved.