FEMTOSECOND TIME-RESOLVED SPECTROSCOPY OF SELF-TRAPPING PROCESSES OF HOLES AND ELECTRON-HOLE PAIRS IN ALKALI BROMIDE CRYSTALS

The dynamics of the lattice relaxation of holes and electron-hole (e-h ) pairs generated by the band-gap excitation in alkali bromide crystal s has been studied in femtosecond time regime at temperature range fro m 6 to 300 K. In KBr including electron-trapping impurities, where rel axation of holes takes place without interaction with electrons, it is found that the self-trapped hole in the form of a halogen-molecular i on the V-K center, is generated via a transient localized-hole center as a precursor. This transient hole center shows broad optical absorpt ion bands in the visible region and has a lifetime of about 3 ps at 27 3 K. It is found that in pure KBr and RbBr, the relaxation process of (e-h) pairs have two distinct stages. In the first stage, which termin ates within 6 ps after excitation, the transient hole centers are gene rated as in doped specimens, and their interaction with electrons resu lts in the fast formation of a Frenkel pair consisting of an F center, a halogen vacancy trapping an electron and an H center, an interstiti al halogen atom. Also, an intermediate state is formed in the first st age, and the state is ascribed to the self-trapped exciton (STE) with the on-center configuration of the basis of its spectroscopic features . The second stage of the relaxation, which lasts over 100 ps at low t emperatures, comprises the off-center relaxation of the on-center STE formed in the first stage into Frenkel pairs and STE's with the off-ce nter configuration. Relaxation pathways and characteristics including temperature dependence are discussed based on the experimental results and their analysis in terms of a rate-equation model.