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

Abstract

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.

The nature of luminescence of KI and KI-Na crystals at low temperature deformation after natural decrease in the symmetry of the lattice

“…Concerning the nature of E x (3.0 eV)-luminescence in KI and KI-Na crystals an important experimental result is obtained -that it becomes dominant and its maximum correlates with an increase in the degree of low-temperature deformation (3.0 eV → 2.63 eV), and not with the concentration of sodium ions in crystals. As a rule, low temperature elastic deformation leads to an increase in intrinsic luminescence due to impurities, which is interpreted by the growth of self-trapping of halogen excitons at regular lattice sites [1][2][3][4][5]. This effect was specially investigated for the KI-Tl crystal and a redistribution of the intensity was observed in favor of intrinsic luminescence with an increase in the degree of elastic deformation [5].…”

“…The luminescence of alkali halide crystals (AHC) is very sensitive to a decrease in the symmetry of the lattice by impurity homologs of various sizes, the degree of elastic and plastic deformation [1][2][3][4][5][6][7]. Cation and anion homologs, as well as vacancies, promote the effective relaxation of electronic excitations around itself, as a result of which we find the luminescence of an self-trapped exciton in the field of point defects [1,2].…”

“…Experiments on low temperature deformation show that the elastic deformation leads to an increase in the probability of radiative annihilation of self-trapped excitons at regular lattice sites because of the shortening of the mean free path of electronic excitations [1][2][3][4]. On the basis of the KI-Tl crystal, the intensity redistribution was favored in favor of intrinsic luminescence with an increase in the degree of elastic deformation [5].…”

Effect of uniaxial strain on the structure of self-localized excitons in alkali halide crystals

“…For all alkali halide crystals in a low-temperature elastic uniaxial strain field, the probability has been established for self-localization of free excitons at regular lattice points, with radiative annihilation due to enhancement of exciton-phonon coupling, leading to rapid shortening of the exciton free path length before self-localization of the excitons [5][6][7].…”

“…For elastic strain, a unique situation is created for studying intrinsic luminescence of alkali halide crystals: all the impurity luminescence occurring in the crystal, characterized by radiative annihilation of SLEs about the impurities, disappears due to shortening of the exciton free path length before their self-localization and as a result emission of SLEs at regular lattice points of the crystal lattice dominates [5].…”