Effect of uniaxial compression on the luminescence of self-trapped excitons in CsI at 80 K

Abstract

01 ~AHC!, in previous work we made use of the method of uniaxial compression proposed and developed by KaplyanskiŽ. In a uniaxially compressed crystalline lattice the symmetry is reduced and the conditions of migration and self trapping of excitations as well as the stability of various configurations of self-trapped excitons ~self-trapped exciton! can vary. The present paper presents the results of a study of the effect of uniaxial compression at 80 K on the luminescence spectra of CsI crystals ~grown at the Estonian Institute of Physics from starting material which was subjected to 60-fold zone melting! excited at 80 K by deep-penetrating x rays (120 kV, 4 mA) and light ~through a VMR-2 monochromator! corresponding to selective creation of excitons ~5.85 eV! and electron–hole pairs ~6.45 eV!. The pressures did not exceed 1 kbar, and the compression was equal to a few percent. Unoriented crystals of CsI were used, which is an approximation to the situation encountered in practice of mounted crystals. All spectra were corrected. For excitation by light with photon energy 5.85 eV, two bands were visible in the luminescence spectra ~curve 1 in Fig. 1!, with maxima at 3.67 and 4.25 eV. The first of these luminescence bands is the well-known self-trapped exciton radiation, associated with the off-configuration. The second is close in the position of its maximum to the intense self-trapped exciton radiation at 4.2 K having the on-configuration, which is assumed to be essentially quenched at 80 K and then flares back up shifted down to 4.1 eV at 300 K ~Refs. 4 and 7!. However, some researchers have also recorded luminescence at 80 K in this spectral region ~the spectra are similar to those presented here!. Such differences point to a high structural sensitivity of the corresponding luminescence centers. For excitation by light with photon energy 6.45 eV, the luminescence spectrum is entirely different ~curve 2 in Fig. 1!: the impurity luminescence and defect luminescence ~in the region 2.0–3.5 eV and around 4.05 eV!, whose intensities are comparable to the luminescence of the off-centers, are well separated. The contribution of the luminescence at 4.25 eV is small. In contrast to this, for excitation by x rays ~curve 1 in Fig. 2! the intensity of the latter luminescence, on the contrary, is higher than the intensity of the luminescence of the self-trapped exciton off-centers, and the impurity luminescence is hardly noticeable. Upon application of uniaxial compression, the lumines-