Partial ionization cross-sections of H2O molecule by 10–25 keV electron ionization

Highlights: •Relative ion fractions of H2O molecule with keV electron impact are measured. •Precursor-specific relative partial ionization cross sections are also measured. •94.4% of single and 5.6% of multiple ionization contribute to the total ion yield. -- Abstract: The relative ionic fractions and the precursor-specific relative partial ionization cross-sections for the fragment ions formed in direct- and dissociative ionization of H2O molecule have been measured for the first time at 10–25 keV electron energies using the multiple ion coincidence imaging technique. We have identified six ions (H2O+, OH+, H+, O+, O2+ and H2+) from the ionization of H2O molecule and have found that the relative ionic fractions for these fragments are almost invariant of impact energy. From multiple-ion coincidence spectra, we have identified three distinct fragmentation channels: namely, H+ + OH+, H+ + O+ + H and H+ + H+ + O resulting from dissociation of H2O2+ and two channels: H+ + O2+ + H and H+ + H+ + O+ resulting from the dissociation of H2O3+. From the analysis, we have estimated that about 94.4% of 'single ionization', 5.3% of 'double ionization' and 0.3% of 'triple ionization' of the parent molecule contribute to the total ion yield. It is further noted that the non-dominant fragment ions O2+ and H2+ possess appreciably larger relative ionic fractions than those obtained at lower impact energies