Picosecond Transient Optical Absorption by Electrons in Molten Alkali Halides

Low concentrations of electrons in molten alkali halides form localized states similar to F-centers. We have investigated the real-time dynamics of such localized states by means of transient optical absorption measurements in molten KCl and KBr. Electron-hole pairs were created by excitation with 30-50 psec pulses at 266 nm, and the resulting absorption was probed at 106 nm. The decay of 1064 nm absorption was followed for 750 psec after initial excitation. The detailed time dependence of the non-geminate recombination process was found to be determined by reactions of the anionic hole centers involving the species X2- and X3-. Analysis of the rate constants confirms that the F-centers electrons are sufficiently mobile to provide the electrical conductivity observed in dilute alkali metal-alkali halide solutions. The results demonstrate the necessity for a polaron-like transport mechanism involving diffusive modes of ionic motion.