Autodissociation of doubly charged water molecules

Abstract

The electron impact dissociative double-ionization cross sections for ${\mathrm{H}}_{2}\mathrm{O}$ between 45 and $1500\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ have been measured using time of flight mass spectrometry. The energy dependence of the ${\mathrm{H}}^{+}+\mathrm{O}{\mathrm{H}}^{+}$ and ${\mathrm{H}}^{+}+{\mathrm{O}}^{+}$ ion pair production cross sections indicate that Auger-like autoionization following a vacancy in the $2{a}_{1}$ molecular orbital is the main double ionization channel at high velocities. In contrast to expectation, these findings show that dissociation through the ${\mathrm{H}}_{2}{\mathrm{O}}^{2+}$ precursor state is a significant process at high collision energies. Knowledge of this process is vital as it has a direct affect on the production of important molecular species, such as ${\mathrm{H}}_{2}$, during water radiolysis. Branching ratios of the various fragments produced following both autoionization and double ionization have also been obtained.