Ultrafast electron transfer processes studied by pump-repump-probe spectroscopy

The photodetachment of Br^-, I^- and OH^- in aqueous solution is studied by 2- and 3-pulse femtosecond spectroscopy. The UV excitation leads to fast electron separation followed by formation of a donor-electron pairs. An additional repump pulse is used for secondary excitation of the intermediates. The 3-pulse technique allows distinguishing the pair-intermediate from the fully separated electron. Using this method we observe a novel geminate recombination channel of.OH with adjacent hydrated electrons. The process leads to an ultrafast quenching (0.7 ps) of almost half the initial number of radicals. The phenomenon is not observed in Br^- and I^-. Our results demonstrate the potential of the 3-pulse spectroscopy to elucidate the mechanism of ultrafast ET reactions.