The photodetachment of aqueous hydroxide (OH(−)(aq) and OD(−)(aq)) is studied using femtosecond pump−probe and pump−repump−probe spectroscopy. The electron is detached after excitation of the hydroxide ion to a charge-transfer-to-solvent (CTTS) state at 202 nm. An early intermediate is observed that builds up within 160 fs and is assigned to nonequilibrated OH−electron pairs. The subsequent dynamics are governed by thermalization, partial recombination, and dissociation of the pairs, yielding the final hydrated electrons and hydroxyl radicals. An additional pulse at 810 nm is used for secondary excitation of the intermediate species so that more insight is gained into the recombination process(es). Using this technique we observe a novel geminate recombination channel of OH with adjacent hydrated electrons. This channel leads to ultrafast quenching (700 fs) of almost half the initial number of radicals. The fast mechanism displays an isotope effect of 1.4 (for OD(−)(aq) quantum yield 35%, time constant 1.0 ps). This process was not observed in similar experiments on aqueous bromide and seems to be related to the special properties of the hydroxide ion and its local H-bonding environment. Our findings underline the high reactivity of the prehydrated electron.

The photodetachment of aqueous bromide after excitation at 202 nm is studied by pump-probe and pump-repump-probe spectroscopy. The initially excited charge-transfer-to-solvent state is followed by an intermediate assigned to non-equilibrated bromine-electron pairs. The subsequent dynamics are governed by equilibration, recombination and dissociation of the pairs, yielding the final hydrated electrons. An additional repump pulse is used for secondary excitation of the intermediate species, increasing the final number of hydrated electrons. Thus, a fraction of the solvent-separated bromine-electron pairs are converted to fully released electrons representing an optical manipulation of the photodetachment pathway. The observed hindrance of the recombination process by repumping allows determination of the effective lifetime of the solvent-separated atom-electron pairs to be 19 +/- 2 ps at room temperature. The measured temperature dependence of the time constant suggests a free energy barrier for pair dissociation of DeltaG = 0.15 +/- 0.02 eV.

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.

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. Photodetachment of aqueous anions studied by two- and three pulse spectroscopy.

We studied the photolysis of neat protonated and heavy water using pump-probe and pump-repump-probe spectroscopy. A novel recombination channel is reported leading to ultrafast quenching (0.7 ± 0.1 ps) of almost one third of the initial number of photo-generated electrons. The efficiency and the recombination rate of this channel are lower in heavy water, 27 ± 5% and (0.9 ± 0.1 ps)(-1), respectively. Comparison with similar data measured after photodetachment of aqueous hydroxide provides evidence for the formation of short-lived OH:e(-) (OD:e(-)) pairs after indirect photoionization of water at 9.2 eV.