Ultra fast relaxation process in alkali halide crystals

Formation mechanism of point defects in alkali halide crystals is introduced from a point of view on the ultra fast relaxation process. Point defects (color centers) are produced in the crystals under irradiation of ionizing radiation or photons with photon energy larger than the band gap energy of the crystals. The first product is the positive hole which is localized in a period of several lattice vibrations to form an X2- molecule (center) in a <110> direction in the crystal. Capture of a conduction electron by the X2- center creates the so-called selftrapped exciton (STE), from which the intrinsic Luminescence and the point defect formation occur. The process starting by the ultra short pulse excitation and ending by the creation of the color centers is an ultra fast phenomenum within a range between several picoseconds and several hundred picoseconds, and gives fundamental knowledge on the defect formation induced by the hole- or electron-lattice interaction. (author)