FEMTOSECOND TIME-RESOLVED SPECTROSCOPY OF THE FRENKEL-PAIR GENERATIONAND SELF-TRAPPED-EXCITON FORMATION IN KCL AND RBCL

The dynamics of the formation of self-trapped excitons (STEs) and of F renkel pairs have been studied in KCl and RbCl upon band-gap excitatio n by femtosecond laser pulses. The relaxation process of photogenerate d electron-hole e-h pairs is found to comprise two distinct stages wit h different natures. The Frenkel pair consisting of an F and an H cent re is formed in the first stage which terminates within a few picoseco nds after excitation, while the STE with the off-centre configuration is formed only at the second stage which follows the first stage and l asts over 20 ps at low temperatures. The relaxation pathways of e-h pa irs and their temperature-dependent changes are examined and discussed on the basis of the experimental results and their analysis in terms of a rare-equation model.