Picosecond Pulse Radiolysis of Direct and Indirect Radiolytic Effects in Highly Concentrated Halide Aqueous Solutions

Recently we measured the amount of the single product, Br-3(-), of steady-state radiolysis of highly concentrated Br- aqueous solutions, and we showed the effect of the direct ionization of Br- on the yield of Br-3(-). Here, we report the first picosecond pulse-probe radiolysis measurements of ionization of highly concentrated Br- and Cl- aqueous solutions to describe the oxidation mechanism of the halide anions. The transient absorption spectra are reported from 350 to 750 run on the picosecond range for halide solutions at different concentrations. In the highly concentrated halide solutions, we the solvated electron is shifted to shorter wavelengths, but its decay, : observed that, due to the presence of Na+, the absorption band of taking place during the spur reactions, is not affected within the first 4 ns. The kinetic measurements in the UV reveal the direct ionization of halide ions. The analysis of pulse-probe measurements show that after the electron pulse, the main reactions in solutions containing 1 M of Cl- and 2 M of Br- are the formation of CIOH-center dot and BrOH-center dot, respectively. In contrast, in highly concentrated halide solutions, containing 5 M of Cl- and 6 M of Br-, mainly Cl-2(-center dot) and Br-2(-center dot) are formed within the electron pulse without formation of CIOH-center dot and BrOH-center dot. The results suggest that, not only Br- and Cl- are directly ionized into Br-center dot and Cl-center dot by the electron pulse, the halide atoms can also be rapidly generated through the reactions initiated by excitation and ionization of water, such as the prompt oxidation by the hole, H2O+center dot, generated in the coordination sphere of the anion.