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 (H 2 O 2+ ) and the emission of two secondary electrons (photoelectron and Auger electron). Our focus lies on detecting and quantifying the superoxide (HO 2 °) production via the direct pathway, i.e., from the reaction between the dissociation product of H 2 O 2+ , i.e., the oxygen atom (∼4 fs), and the °OH radicals present in the secondary electron tracks. The HO 2 ° yield for 1620 eV photons, via this reaction pathway, was found to be 0.005 (±0.0007) μmol/J (formed within the ∼ps range). Experiments were also performed to determine the yield of HO 2 ° production via another (indirect) pathway, involving solvated electrons. The indirect HO 2 ° 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.