by Anna Balcerzyk, Uli Schmidhammer, Furong Wang, Aurélien de la Lande and Mehran Mostafavi

Picosecond pulse radiolysis measurements have been performed in several highly concentrated HClO₄ and H₃PO₄ aqueous solutions containing silver ions at different concentrations. Silver ion reduction is used to unravel the ultrafast reduction reactions observed at the end of a 7 ps electron pulse. Solvated electrons and silver atoms are observed by the pulse (electron beam)–probe (supercontinuum light) method. In highly acidic solutions, ultrafast reduction of silver ions is observed, a finding that is not compatible with a reaction between the H^• atom and silver ions, which is known to be thermally activated. In addition, silver ion reduction is found to be even more efficient in phosphoric acid solution than that in neutral solution. In the acidic solutions investigated here, the species responsible for the reduction of silver atoms is considered to be the precursor of the H^• atom. This precursor, denoted (e^–, H₃O⁺), is a pair constituting an electron (not fully solvated) and H₃O⁺. Its structure differs from that of the pair of a solvated electron and a hydronium ion (e_s^–, H₃O⁺), which absorbs in the visible region. The (e^–, H₃O⁺) pair , called the pre-H^• atom here, undergoes ultrafast electron transfer and can, like the presolvated electron, reduce silver ions much faster than the H^• atom. Moreover, it is found that with the same concentration of H₃O⁺ the reduction reaction is favored in the phosphoric acid solution compared to that in the perchloric acid solution because of the less-efficient electron solvation process. The kinetics show that among the three reducing species, (e^–, H₃O⁺), (e_s^–, H₃O⁺), and H^• atom, the first one is the most efficient.