Using X-ray luminescence spectra, the enhancement of both σ-luminescence of self-trapped excitons and Ех luminescence (band maxima at 3.89 and 3.1 eV, respectively) has been revealed for the first time in RbI single crystals exposed to elastic uniaxial stress at 93 K. The intensity of these emissions increases linearly with the relative degree of applied uniaxial deformation of the crystal up to ɛ = 1%, while the luminescence undergoes saturation at higher values of ɛ. The behavior of the I = f (ɛ) dependence for σ and Ех emissions suggests that Ех luminescence is intrinsic, and is connected with the radiative relaxation of self-trapped excitons in the field of local deformation of a RbI regular lattice.
The effect of low-temperature uniaxial deformation on the self-trapping-limited mean free path of excitons in a KI–Tl crystal was revealed from x-ray luminescence spectra. The analysis of the dependence of the intensity ratio of the Tl-center emission (2.85 eV) and the luminescence of self-trapped excitons (π-component; 3.3 eV) on the extent of low-temperature deformation showed that in the KI–Tl crystal (3 × 10−3 mol. %) the self-trapping-limited mean free path of excitons is comparable with the distance between Tl atoms (20–27)a under a deformation ε = 2%. As the compression increases to ε ≥ 2%–5%, the mean free path drops to (27-5.35)a. The results of modeling based on the continuum approximation showed that with increasing temperature and the degree of low-temperature deformation the height of the potential barrier for the exciton self-trapping drops, which is consistent with the reduction of the mean free path of excitons in the KI–Tl crystal.
The X-ray luminescence spectra at low temperature (90K) uniaxial deformation were researched for KI and KI-Na crystals, that were stored for more than 10 years. The following regularities were observed: firstly, the intensities of the intrinsic emission bands at 3.3 eV ( π ) and 4.1 eV ( σ ) become equal in comparison with freshly grown crystals; and secondly, with an increase in the degree of low-temperature uniaxial deformation, a gradual shift of the emission spectra occurs in two directions: the luminescence maximum at 3.3 eV shifts towards short wavelengths, the final position is fixed at 3.9 eV, which practically merges with σ -luminescence; the radiation maximum at 3.0 eV (Ex-radiation) is shifted toward long wavelengths, the final position which is fixed at 2.8 eV. Thus, low temperature uniaxial deformation leads to the separation of the emission spectra at 3,3 eV( π ) → 3,9 eV and 3.0 eV ( E x ) → 2,8 eV, which are interpreted by the separation of weak → on and weak → strong exciton configurations , respectively.
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