The primary steps of a one-electron reduction of a disulfide molecule (cystamine) by the infrared
p-like state of an excited hydrated electron have been investigated by femtosecond laser spectroscopy. The
initial electron photodetachment from chloride ion or phenothiazine is triggered by a two-photon ultraviolet
excitation. In homogeneous aqueous cystamine solution, a subpicosecond univalent reduction of the disulfide
molecule by an infrared prehydrated electron competes with the nonadiabatic relaxation of trapped electrons
(electron solvation process). This presolvation one-electron reduction occurs with a characteristic time of 160
± 20 fs at 294 K. Within the electron solvation regime, this elementary redox process is totally achieved in
less than 1 × 10-12 s and exhibits a probability 9 times higher than the radiationless relaxation of an infrared
excited electron. In aqueous organized assemblies (cationic micelles) the partitioning of reactants does not
influence the frequency rate of a prehydration reaction but modifies the early branching between reactive and
nonreactive IR electronic channels. The real-time UV probing of a nascent sulfur-centered radical anion
(RS∴SR-)aq is discussed in the framework of a two-center-three-electron bond.
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