Superoxide Production under Soft X-ray Irradiation of Liquid Water

Soft X-rays behave like particles with high linear energy transfer, as they deposit a large amount of their energy in the nanometric range, triggered by inner-shell ionization. In water, this can lead to the formation of a doubly ionized water molecule (H2O2+) and the emission of two secondary electrons (photoelectron and Auger electron). Our focus lies on detecting and quantifying the superoxide (HO2 degrees) production via the direct pathway, i.e., from the reaction between the dissociation product of H2O2+, i.e., the oxygen atom (similar to 4 fs), and the degrees OH radicals present in the secondary electron tracks. The HO2 degrees yield for 1620 eV photons, via this reaction pathway, was found to be 0.005 (+/- 0.0007) mu mol/J (formed within the similar to ps range). Experiments were also performed to determine the yield of HO2 degrees production via another (indirect) pathway, involving solvated electrons. The indirect HO2 degrees yield, measured experimentally as a function of photon energy (from 1700 to 350 eV), resulted in a steep decrease at around 1280 eV and a minimum close to zero at 800 eV. This behavior in contradiction with the theoretical prediction reveals the complexity hidden in the intratrack reactions.