Femtosecond time-resolved spectroscopy of the Frenkel-pair generation and self-trapped-exciton formation in KCl and RbCl

The dynamics of the formation of self-trapped excitons (STEs) and of Frenkel pairs have been studied in KCl and RbCl upon band-gap excitation by femtosecond laser pulses. The relaxation process of photogenerated electron-hole e-h pairs is found to comprise two distinct stages with different natures. The Frenkel pair consisting of an F and an H centre is formed in the first stage which terminates within a few picoseconds after excitation, while the STE with the off-centre configuration is formed only at the second stage which follows the first stage and lasts over 20 ps at low temperatures. The relaxation pathways of e-h pairs 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.