Femtosecond spectroscopic studies of self-trapping processes of free and interacting electron-hole pairs in RbI

The initial formation processes of self-trapped exciton (STE) have been investigated by means of femtosecond pump and probe spectroscopy in RbI changing excitation photon energies. Under the above gap excitations the STEs are formed via a one-center self-trapped state, while the STEs are directly formed for the resonant excitation of 2p-free excitons. In the case of band-edge excitations, both formation pathways are observed and the formation time is dependent on the excitation photon energies. These results demonstrate the existence of the potential barrier between the free state and self-trapped state.