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

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 10–25keV 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.