Mechanoluminescence induced by elastic deformation of coloured alkali halide crystals using pressure steps

Abstract

During the elastic deformation of coloured alkali halide crystals, the bending segments of dislocations capture F-centre electrons lying in the expansion region of edge dislocations, to the states of dislocation band. After the separation from interacting F-centres, the captured electrons move together with the bending segments of dislocations and also drift along the axis of dislocations and subsequently the radiative electron–hole recombinations, owing to both the processes of captured-electron movement, give rise to the light emission. The generation rate of electrons in the dislocation band and the mechanoluminescence (ML) intensity initially increase with time, attain maximum value at a particular time, and then they decrease with time. The intensity I m corresponding to the peak of ML intensity versus time curve and the total intensity I T of ML increase with the applied pressure and also with the density of F-centres in the crystals. At low temperature, both I m and I T increase with temperature and at higher temperature they decrease with increasing temperature due to the thermal bleaching of F-centres and also due to the decrease in luminescence efficiency. Thus, both I m and I T are optimum for a particular temperature of the crystals. For longer time duration, the ML intensity decreases exponentially with time in which the decay time is equal to the lifetime of interacting F-centres. Expressions derived for the different characteristics of ML are able to explain the experimental results. It is shown that the time constant for rise of pressure, lifetime of the interacting F-centres or damping time of dislocation segments, and the activation energy can be determined from the ML measurements.