Time resolved spectroscopy for radiation damage processes induced by electronic excitation in insulators

We review current studies by means of femtosecond time resolved spectroscopy on defect formation induced by valence-electron excitation in halide crystals. We first summarize the characteristics of several transient relaxed configurations of electron-hole pairs or excitons which play important roles in the defect formation process. Then, the results on the self-trapping process of holes in this material are described to show the specific feature revealed by the femtosecond spectroscopy. Finally, the dynamics of the Frenkel pair formation in alkali halide crystals is discussed based on the results obtained for several crystals at wide temperature range from 5 to 300 K. Emphasis is placed on the new feature of damage mechanism which rakes place during the lattice relaxation of the localized electron-hole pairs.