Electron detachment and recombination in aqueous solutions studied with 2- and 3-pulse femtosecond spectroscopy

The electron photodetachment of the aqueous halides and hydroxide is studied after excitation in the lowest CTTS state. The initially excited state is followed by an intermediate assigned to a donor-electron pair that displays a competition of recombination and separation. The geminate dynamics measured in the various CTTS systems reveal a strong influence of the parent radical. The electron survival probability decreases significantly from 0.77 to 0.29 going from F- to OH^-. Results for I^- show that excitation of a higher-lying CTTS state opens a new relaxation channel, which directly leads to fully hydrated electron, while the relaxation channel discussed above is not significantly affected. Using pump-repump-probe spectroscopy the pair species is verified via a secondary excitation with separation of the pairs so that the yield of released electrons is increased. Three pulse spectroscopy on aqueous hydroxide give evidence for an additional ultrafast (∼700 fs) geminate recombination channel in this system. Comparison of these data with similar experiments on neat water after two-photon excitation with total energy of 9.2 eV demonstrates the important role of (OH^-)* for the indirect photoionization of water.