Effect of Low-Temperature Plastic Deformation upon the Optical Properties of Alkali Halides

The optical absorption spectrum of KBr and KI crystals, which have been plastically deformed at low temperature, has been observed in the region of the $\alpha$ band in order to detect single-ion vacancies generated by moving dislocations. As a result of the present experiment no $\alpha$ band has been observed in KBr and KI crystals plastically deformed at several temperatures between -180°C and -125°C. The number of $\alpha$ centers is estimated to be less than ${10}^{16}$ ${\mathrm{cm}}^{-3\mathrm{}}$ for KBr or KI crystals after 5% of plastic strain. A general broadening of the fundamental absorption band takes place after plastic deformation. On the other hand, a yellow photoluminescence can be excited in the spectral region of the broadened fundamental absorption. A rough proportionality exists between the amount of luminescence and the number of photons absorbed in the region of the crystal responsible for the extra absorption. Annealing at room temperature reduces both luminescence and extra absorption. A qualitative explanation is given, which is based on the present knowledge of defects in alkali halides.