Ultrafast delocalization, excited-state chemistry and spectroscopy of the hydrated electron
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Femtosecond spectroscopic measurements are reported on the delocalization, electron transfer reactions, solvation dynamics, and spectroscopy of the hydrated electron (eaq- ). The eaq- is an excess electron trapped in water and it has been under intense investigations for decades, e.g. for its charge transfer reactions, and nonadiabatic electronic transition. In the research presented in this dissertation, we choose to study eaq- because it is a unique prototype for studying localization and delocalization of a charge in water, electron transfer kinetics, solvation dynamics, and spectroscopy involving a highly delocalized excited state. First, the delocalization of eaq- is observed using a novel spatially sensitive technique which involves monitoring the effect of photo-excitation on the electron/hole geminate recombination following photoionization of neat water. vi The results indicate that excitation to the highly delocalized conduction band (CB) state produces significant suppression of geminate recombination, whereas excitation to the localized excited state (p-state) does not. The results show that the spatial extent of CB has a lower limit of 30 Å. Secondly, the electron transfer reactions, which correspond to scavenging of the various delocalized excited and precursor states of the hydrated electron by localized electron acceptors such as Cd2+, are studied. The rate constant of electron transfer per scavenger is found to inversely proportional to the volume of the specific state. Thirdly, the solvation dynamics of the different precursors and excited states of eaq- are investigated. The results show that the timescale of solvation depends on the initial degree of delocalization of the specific state. Fourthly, using the spatially sensitive technique with multi-color excitation, the broad and featureless absorption spectrum of eaq- is decomposed into two contributions. The first contribution involves the transition from the localized ground state (s-state) to the p-state and the second contribution corresponds to the transition from the s-state to CB. Finally, in order to obtain further insights on the spatial extent of CB, photodetachment experiments are performed on the hydrated electron trapped in the water pool of a reverse micelle (RM) with a radius ranging from 18 to 45 Å. The results indicate that the spatial extent of CB is comparable to the sizes of RM.