Photolysis of water-radical ions H2O+in the xuv: Fragmentation through dicationic states

Abstract

The photofragmentation of the water cation H${}_{2}$O${}^{+}$ through dicationic states has been studied at $35.0\ifmmode\pm\else\textpm\fi{}0.2$ nm ($35.4\ifmmode\pm\else\textpm\fi{}0.3$ eV) and $21.8\ifmmode\pm\else\textpm\fi{}0.2$ nm ($56.8\ifmmode\pm\else\textpm\fi{}0.5$ eV) with a crossed ion-photon beams experiment at the free electron laser FLASH. The dissociation of the dications is found to be similar at the two wavelengths and to proceed into ${\mathrm{O}}^{0}+2{\mathrm{H}}^{+}$, ${\mathrm{OH}}^{+}+{\mathrm{H}}^{+}$, and ${\mathrm{O}}^{+}+{\mathrm{H}}_{2}{}^{+}$, with determined ratios ${\ensuremath{\sigma}}_{{\mathrm{OH}}^{+}+{\mathrm{H}}^{+}}/{\ensuremath{\sigma}}_{{\mathrm{O}}^{+}+{\mathrm{H}}_{2}{}^{+}}=4.2\ifmmode\pm\else\textpm\fi{}0.3$ and ${\ensuremath{\sigma}}_{{\mathrm{OH}}^{+}+{\mathrm{H}}^{+}}/{\ensuremath{\sigma}}_{{\mathrm{O}}^{0}+2{\mathrm{H}}^{+}}>0.7$. The measured kinetic-energy releases for these processes are consistent with three-body breakup (${\mathrm{O}}^{0}+2{\mathrm{H}}^{+}$) occurring mainly through the $2\phantom{\rule{0.28em}{0ex}}{}^{3}\phantom{\rule{-0.16em}{0ex}}{A}^{\ensuremath{'}\ensuremath{'}}$ and $2\phantom{\rule{0.28em}{0ex}}{}^{1}\phantom{\rule{-0.16em}{0ex}}{A}^{\ensuremath{'}\ensuremath{'}}$ states of ${\mathrm{H}}_{2}{\mathrm{O}}^{2+}$ and two-body breakup (${\mathrm{OH}}^{+}+{\mathrm{H}}^{+}$) occurring through $X\phantom{\rule{0.28em}{0ex}}{}^{3}\phantom{\rule{-0.16em}{0ex}}{A}^{\ensuremath{'}\ensuremath{'}}$, $1\phantom{\rule{0.28em}{0ex}}{}^{3}\phantom{\rule{-0.16em}{0ex}}{A}^{\ensuremath{'}}$, and $1\phantom{\rule{0.28em}{0ex}}{}^{1}\phantom{\rule{-0.16em}{0ex}}{A}^{\ensuremath{'}\ensuremath{'}}$ states of H${}_{2}$O${}^{2+}$, as predicted in a recent theoretical study [Gervais et al., J. Chem. Phys. 131, 024302 (2009)]. In addition to the kinetic-energy releases, we also report on fragment correlation in the three-body channel where the two protons carry the major part of the released momentum.