Femtosecond time-resolved spectroscopy of the formation of self-trapped excitons in CaF2

Self-trapped excitons (STE's) in CaF2 are formed via two distinct processes after band-gap excitation by femtosecond light pulses at low temperatures; the fast process terminates within a few ps after excitation, and the slow process that follows the former lasts over 20 ps. The fast formation of STE's is associated with the decay of the precursor, which shows a broad absorption in visible region. The relaxation pathways of electron-hole pairs are discussed based on the experimental results and their analysis in terms of a rate-equation model