Unexpected Ultrafast Silver Ion Reduction: Dynamics Driven by the Solvent Structure

Picosecond pulse radiolysis measurements have been performed in neutral and highly acidic aqueous solutions containing silver ions at different concentrations. Silver ion reduction is used to understand the ultrafast chemistry of irradiated water and aqueous solutions. The absorption band measured at the end of the 7-ps electron pulses has an intense band with a maximum at 360 nm due to the formation of silver atoms. Kinetics shows that the amount of silver atom formed at the end of the electron pulse in phosphoric acid solutions is greater than that in neutral water. This unexpectedly high yield of silver atom formation cannot be explained solely by the reaction between silver ions and solvated electrons in neutral solutions nor by the reaction with hydrogen atoms in phosphoric acid solutions. To explain the observed ultrafast reduction of silver ions, the presolvated electron, be it free or paired to the hydronium cation, must react very quickly with a silver ion, potentially competing with geminate recombination of the electron and its sibling radical cation.