Spur Reactions Observed by Picosecond Pulse Radiolysis in Highly Concentrated Bromide Aqueous Solutions

The formation of the well-known product Br3(-), observed in the steady-state radiolysis of highly concentrated Br(-) aqueous solutions, has now been directly observed at ultrashort times corresponding to the relaxation of the spur. The transient absorption induced by picosecond pulse radiolysis of 6 M Br(-) aqueous solution was probed simultaneously at 260 nm with the third harmonic laser wave and from 350 to 750 nm with a supercontinuum generated by the fundamental laser wave. This approach allows several transient radiolytic species to be followed in parallel, particularly the solvated electron, BrOH(-•), Br2(-•), and Br3(-). The kinetics measured within 4 ns at 260 and 370 nm clearly exhibit that the decay of Br2(•-) is correlated with the formation of Br3(-). In highly concentrated Br(-) solutions, the OH(•) radical is fully replaced by Br2(•-), and the spur kinetics of OH(•) radical in pure water is comparable with that of Br2(-•). Model calculations indicate that the main OH(•) radical combination product H2O2 in pure water has formation kinetics similar to that of Br3(-) in 6 M Br(-) solutions. Moreover, they point out that oxidation of Br(-) occurs within the electron pulse both by direct energy absorption and by scavenging of the water radical cation, H2O(•+).