Competition Reactions of H2O•+ Radical in Concentrated Cl- Aqueous Solutions: Picosecond Pulse Radiolysis Study

Picosecond pulse-probe radiolysis measurements of highly concentrated Cl- aqueous solutions are used to probe the oxidation mechanism of the Cl-. The transient absorption spectra are measured from 340 to 710 nm in the picosecond range for the ultrafast electron pulse radiolysis of halide solutions at different concentrations up to 8 M. The amount of Cl-2(center dot-) formation within the electron pulse increases notably with increasing Cl- concentration. Kinetic measurements reveal that the direct ionization of Cl- cannot solely explain the significant amount of fast Cl-2(center dot-) formation within the electron pulse. The results suggest that Cl- reacts with the precursor of the OH center dot radical, i.e., H2O center dot+ radical, to form Cl-center dot atom within the electron pulse and the Cl-center dot atom reacts subsequently with Cl- to form Cl-2(center dot-) on very short time scales. The proton transfer reaction between H2O center dot+ and the water molecule competes with the electron transfer reaction between Cl- and H2O center dot+. Molecular dynamics simulations show that number of water molecules in close proximity decreases with increasing concentration of the salt (NaCl), confirming that for highly concentrated solutions the proton transfer reaction between H2O center dot+ and a water molecule becomes less efficient. Diffusion-kinetic simulations of spur reactions including the direct ionization of Cl- and hole scavenging by Cl show that up to 30% of the H2O center dot+ produced by the irradiation could be scavenged for solutions containing 5.5 M Cl-. This process decreases the yield of OH center dot radical in solution on the picosecond time scale. The experimental results for the same concentration of Cl- at a given absorbed dose show that the radiation energy absorbed by counterions is transferred to Cl- or water molecules and the effect of the countercation such as Li+, K+, Na+, and Mg2+ on the oxidation yield of Cl- is negligible.