Electron detachment from anions in aqueous solutions studied by two- and three-pulse femtosecond spectroscopy

The electron photodetachment of the aqueous halides and hydroxide is studied after resonant excitation in the lowest charge-transfer-to-solvent (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. Using pump-repump-probe (PREP) spectroscopy, the pair species is verified via a secondary excitation with separation of the pairs so that the yield of released electrons is increased. The observed recombination process on the one hand and the similar absorptions of the intermediate and the hydrated electron on the other hand suggest that the donor-electron pairs incorporate only few if not just one water molecule. 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-. The extracted dissociation rates of the halogen-electron pairs seem to be proportional to the mutual diffusion coefficients of the geminate particles, while such a relation between the recombination rate and the diffusion coefficient is not found. Results for I- show that excitation of a higher-lying CTTS state opens a new relaxation channel, which directly leads to a fully hydrated electron, while the relaxation channel discussed above is not significantly affected.