Double ionization of water molecules induced by swift protons

Experimental cross sections for single and double ionization of H₂O by swift H⁺ with energy ranging from 0.3 to 2.0 MeV are reported. In this energy range the ionization is the dominant collision process and charge transfer reactions can be disregarded. A multihit coincidence technique is used to measure the H⁺+OH⁺ and H⁺+O⁺ fragmentation channels. Single- and double-hit differential measurements together with a semiempirical calculation allow separating quantitatively the prompt and Auger-like decay contributions to fragmentation following a vacancy in the 2 a₁ molecular orbital. Concerning the double-ionization channel, it is found that for lower energies the mechanism of a sequential double-electron removal, known as TS2, dominates. For energies above above 750 keV/u ionization resulting from a single vacancy followed by an Auger like deexcitation takes over the TS2, becoming the main contribution to the double-ionization cross section. Our results are compared to the electron-impact data within the same velocity range and also with theoretical calculations available in the literature.