The 185 nm photolysis of 1,4-dioxan in aqueous solution in the presence of N 2O:N 2 formation via dioxan photoionization or photoexcitation?

Abstract

In the photolysis (185 nm) of 1,4-dioxan in N 2O-saturated neutral aqueous solutions, where all the photolysing light is absorbed by dioxan, nitrogen is formed with a quantum yield of unity. On reducing the N 2O concentration, Φ(N 2) decreases, and at an N 2O concentration of about 1 × 10 −3 M it reaches half its original value. A comparison of these results with data from the γ radiolysis of the solutions, where the freely diffusible electron is formed, indicates that this species does not give rise to the photolytic formation of nitrogen. The lifetime of the nitrogen-forming species generated by photolysis is about 0.1 μs; this is much shorter than that of the freely diffusible hydrated electron, but much longer than that of a 1,4-dioxan singlet excited state. This suggests that the nitrogen-forming species may be an ion pair where, on relaxation of the radical cation, the neutralization reaction is slowed down because the configuration has become unfavourable. The hypothesis that the nitrogen-forming species may be a triplet state is also considered.