Scavenging the Water Cation in Concentrated Acidic Solutions

Picosecond pulse radiolysis techniques were used to observe the kinetics of the SO4 center dot-, H2PO4, Cl-2(center dot-), and Br-2(center dot-) species formed in the fast oxidation of concentrated and highly acidic solutions of SO42-, PO43-, Cl-, and Br-. Experimental results were compared with model predictions to gain insight into the possible mechanisms occurring on the fast time scales. Simple kinetics involving the oxidizing OH center dot radical formed by radiolytic water decomposition could not account for the observed yields at the very short times (within the electron pulse similar to 7 ps). Diffusion-kinetic simulations of the spur reactions induced by the incident electrons show that additional oxidation of the solutes must occur at very short times and involves their direct ionization along with scavenging of the highly oxidizing H2O center dot+ radical formed in the initial ionization of the water medium. The fraction of H2O center dot+ radicals scavenged varies as 0.26, 0.68, 0.92, and 0.97 for PO43-, SO42-, Cl-, and Br- solutions, respectively. These studies represent the first semiquantitative estimation of the H2O center dot+ radicals scavenging fractions for such a wide range of solutes.