Two slow decay components are found in the low-temperature decay kinetics of the A, emission of Ga+ and In+ centres in alkali halides. The emission, excitation, and polarization spectra of these components as well as the temperature dependences of their decay times and light sums and the influence of the electric and magnetic fields on the decay kinetics of the polarized A T emission are carefully investigated. It is shown that the observed effects are caused by an off-centre position of Ga+ and In+ ions in the triplet relaxed excited state (RES). The off-centre displacement occurs most probably in one of the (100) directions perpendicular to the tetragonal Jahn-Teller distortion axis. Rapid tunnelling transitions between various energetically equivalent off-centre positions of the impurity ion lead to the tunnel splitting of each metastable tetragonal (T) minimum of the triplet RES. Both the observed slow decay components of the AT emission are connected with the radiative decay of the two lower states. The orthorhombic structure of the A T RES, found by the technique of optical detection of electron spin resonance (OD ESR) several years ago for Gat centres in some alkali halides, is explained here as another evidence of the same phenomenon.

The AT luminescence of KCl: Ga is investigated at 0.38 to 100 I< by spectral, kinetic, and polarization methods. At extremely low temperatures the luminescence decay curve in the magnetio field is decomposed into three components (fast, quick, and slow) with decay times dependent, but light sums independent of the field strength. Polarization degrees of the components are independent both of field and time. The experimental results are discussed in terms of a model suggesting a strong interaction of the optical electron of Ga+ with tetrngonnl vibrations of the crystal lattice.

The dependence of electronic wave functions on the coordinates of not totally symmetric vibrations and the hyperfine interaction are considered theoretically as the possible reasons of the radiative decay of the metastable minima of the sp excited state of Sn2+v, centres in KCI crystals. It is shown that the dependence of electronic wave functions on the coordinates of not totally symmetric vibrations, which leads to the dynamical lowering of symmetry and removal of forbiddenness for radiative transition from metastable minima (deviation from the Condon approximation), is the main reason of the radiative decay. Taking into account also the mixing ofthe functions of the radiative and metastable minima by the hyperfine interaction, a 1 to 10% shortening of decay time t is expected at the transition from natural tin (containing 16% of Sn1la) to pure isotop Sn119. A comparison of the decay times is performed which are measured at 2 and 4.2 I< for A T luminescence of Sn2+vc centres in KCI crystals doped with tin having different isotopic abundance. The values of z in KCI: SnCI, and KCI: llSSnC1, crystals are shown to differ to about 4y0, whereas in KCI crystals doped with the isotope ll9Sn the decay time is always shorter than that in KCl with natural tin. This result, being the first experimental observation of the influence of the hyperfine interaction on luminescence decay kinetics of impurity centres in the crystals, is in good agreement with the theory. 3aBHCHMOCTb 3JIeHTPOHHbIX BOJlHOBbIX