Scavenging of "dry" electrons prior to hydration by azide ions: effect on the formation of H₂ in the radiolysis of water by ⁶⁰Co γ-rays and tritium β-electrons

In this study, we use Monte Carlo track chemistry simulations to show that "dry" secondary electrons, precursors of the "hydrated" electron (e^-_aq), can be scavenged on the sub-picosecond time scale prior to hydration, by a high concentration (>0.1–1 M) of azide ions (N₃^- ) in water irradiated with ⁶⁰Co γ-rays and tritium β-electrons at 25 °C. This is a striking result, as N₃^- is known to react very slowly with e_aq. These processes tend to significantly reduce the yields of H₂ as observed experimentally. For both energetic Compton electrons ("linear energy transfer", LET 0.3 keV/lm), which are generated by the cobalt-60 γ-rays, and 3 H β-electrons (LET 6 keV/lm), our H₂ yield results confirm previous Monte Carlo simulations, which indicated the necessity of including the capture of the precursors to e_aq. Interestingly, our calculations show no significant changes in the scavenging of "dry" electrons at high azide concentrations in passing from γ-radiolysis to tritium β-radiolysis (i.e., with LET). This led us to the conclusion that the higher H₂ yield observed experimentally for ³H β-electrons compared with ⁶⁰Co γ-rays is mainly explained by the difference in the radiation track structures during the chemical stage (>1 ps). The higher LET of tritium β-electrons leads to more molecular products (H₂ in this case) in tritium radiolysis than in γ-radiolysis. Finally, a value of ~0.5 nm was derived for the reaction distance between N₃^- and the "dry" electron from the H² yields observed in ⁶⁰Co γ-radiolysis at high N₃^- concentrations. (author)