A STUDY OF ULTRAFAST SELF-TRAPPING PROCESSES OF ELECTRON-HOLE PAIRS IN RBI BY FEMTOSECOND PUMP AND PROBE SPECTROSCOPY

Initial formation processes of self-trapped excitons from free electron-hole pairs in RbI crystals have been investigated by means of a femtosecond pump and probe spectroscopy. The short-lived intermediate analogous to the one-center self-trapped hole (STH) has been observed as a precursor for the self-trapped exciton (STE) in the time region of 0.3-5 ps. The formation process of the STE in RbI is found to be quite similar to that observed in KI rather than KBr. From the observation of the two types of formation process depending on the species of halogen atom, it is found that the lattice relaxation leading to the STE formation is affected by the width of the valence band which consists of p-orbitals of halogen ions.