Study of the self-trapped exciton andF-center formation on a picosecond time scale in KBr

We have studied optical absorption at 694 nm in KBr, on a picosecond time scale, after two-photon excitation by a 347-nm pulse. We have measured an $F$-center rise time of (15 ± 10) psec in the temperature range 300-660 K. The self-trapped exciton exhibits more complicated creation kinetics, with population of its lower state delayed by about 60 psec. In addition, we discuss a model of $F$-center production involving the free exciton as the precursor state.