Direct Evidence for Transient Pair Formation between a Solvated Electron and H₃O⁺ Observed by Picosecond Pulse Radiolysis

The reaction between the solvated electron and hydronium cation H₃O⁺ in water constitutes a fundamental reaction in chemistry. Due to significant rearrangement of solvent molecules around both the electron and H₃O⁺, the reaction rate of this process is not controlled by diffusion. The presence of a reaction barrier suggests the formation of an intermediate that has so far not been observed. Here, the time-resolved visible absorption spectra in three concentrated acid solutions, perchloric, sulfuric, and phosphoric, at various concentrations are recorded by the picosecond pulse radiolysis method. In contrast to previous reports, a strong blue shift of the absorption band of the solvated electron in acidic solutions compared to neat water is clearly observed, consistent with formation of a pair between the solvated electron and hydronium cation