Rate and reaction pathway of the degradation of organic compounds in aqueous solution by VUV-irradiation (Xe-excimer: 172 nm) are strongly influenced by the concentration of dissolved molecular oxygen in the volume of primary reactions. Due to the very short lifetimes of the intermediates generated by the homolysis of water, this volume is almost identical with the irradiated fraction of the total reactor volume. Given the fact, that VUV-radiation is totally absorbed within less than 0.1 mm due to the high absorption cross-section of water, investigations on the concentration of dissolved molecular oxygen in the volume of primary reactions require a very high spatial resolution. This spatial resolution is reached by using a very small oxygen optode connected to the photoreactor. The measurements show that VUV-irradiation of a homogenous aqueous system is leading to pronounced heterogeneity: a very thin volume, close to the surface of the light source, which is characterised by the diffusion controlled reactions of primary radicals of very short lifetimes with the organic substrate and the trapping of the C-centered radicals by molecular oxygen, may be differentiated from the remaining major part of the reactor volume in which slower thermal reactions initiated by peroxyl radicals dominate.

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