We have observed the formation of OH radicals in aqueous solutions under the effect of 253.7 nm (4.89 eV) UV radiation delivered from a low-pressure Hg-lamp . By combination of chemical actinometry (iodide-iodate system) with the tools conventionally used for the characterization of radical yields during radiolysis of aqueous solutions (a fluorescence dosimeter - coumarin-3-carboxylic acid - and the traditional ferrous sulfate dosimeter) we determined the part of radiant energy efficiently absorbed to contribute to the formation of OH radicals via the photolysis of water. The effective quantum yield of the OH radical formation was determined to be approximately 0.08. A combination of at least two 4.89 eV photons would be needed to deliver enough energy for direct H-OH dissociation (that needs between at least 5.0 and 5.4 eV to take place). Since direct two-photon effects can be considered as less probable in the case of the Hg-lamp, we suggest a mechanism based on the interaction of two long-living excited states of water molecules. Such an interaction could provide the energy required for the dissociation of one water molecule into OH and H radicals.
 K. Tomanová et al.: At the crossroad of photochemistry and radiation chemistry: formation of hydroxyl radicals in diluted aqueous solutions exposed to ultraviolet radiation, Phys. Chem. Chem. Phys. 19, 29402 (2017).