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Relaxation Mechanism of the Hydrated Electron

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Science  20 Dec 2013:
Vol. 342, Issue 6165, pp. 1496-1499
DOI: 10.1126/science.1246291

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Relaxing in a Water Jet

High-energy irradiation of liquid water and its solutes can transiently liberate electrons, which act as potent chemical reductants, but they are challenging to characterize precisely. Seeking to bridge the gap between liquid and gas, Elkins et al. (p. 1496) report results from photoelectron spectroscopy of hydrated electron dynamics in a liquid jet. The results reveal a very rapid transition from the electronic excited state to the ground state, prior to full relaxation of the solvent shell.

Abstract

The relaxation dynamics of the photoexcited hydrated electron have been subject to conflicting interpretations. Here, we report time-resolved photoelectron spectra of hydrated electrons in a liquid microjet with the aim of clarifying ambiguities from previous experiments. A sequence of three ultrashort laser pulses (~100 femtosecond duration) successively created hydrated electrons by charge-transfer-to-solvent excitation of dissolved anions, electronically excited these electrons via the sp transition, and then ejected them into vacuum. Two distinct transient signals were observed. One was assigned to the initially excited p-state with a lifetime of ∼75 femtoseconds, and the other, with a lifetime of ∼400 femtoseconds, was attributed to s-state electrons just after internal conversion in a nonequilibrated solvent environment. These assignments support the nonadiabatic relaxation model.

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