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The luminescence and excitation spectra of electronic excitations localized near a single anion vacancy (e,), a divacancy (ed), and a vacancy quartet (e,) are investigated. The shifts of spectral characteristics of these excitations with respect to those of the excitations in regular lattice sites (e,) have regular behavior. It turned out that the charge of vacancy defects is the main perturbing factor for e,, ed, and eq creation and the dimension of vacancy defects is the main perturbing factor for the radiative decay em, ed, and e,. The peculiarities of e,, ed, and e, relaxation are discussed in terms of strong and weak off-centers for e, in KBr and KI, respectively.') Riia 142, EE 2400 Tartu, Estonia. ' ) Pr. Moldagulovoi 34, 463 019 Aktyubinsk, Kazak.

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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-toband 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 E x /π and, particularly, of the σ/E x 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.