Photoionization and fragmentation of H3O+under XUV irradiation

Abstract

The photolysis of the hydronium cation H${}_{3}$O${}^{+}$ has been studied at the extreme ultraviolet wavelengths of $35.56\ifmmode\pm\else\textpm\fi{}0.24$ nm ($34.87\ifmmode\pm\else\textpm\fi{}0.24$ eV) and $21.85\ifmmode\pm\else\textpm\fi{}0.17$ nm ($56.74\ifmmode\pm\else\textpm\fi{}0.44$ eV) using a crossed ion-photon beam setup at the free-electron laser FLASH. Coincidence photoelectron and photofragment spectroscopy was performed at 21.85 nm, where both inner and outer valence ionization are allowed, and revealed that the XUV photolysis of H${}_{3}$O${}^{+}$ is by far dominated by ionization of outer valence electrons forming the ${}^{1}{A}_{1}$ and ${}^{2}E$ states of the dication H${}_{3}$O${}^{2+}$. The dications were found to dissociate into the channels H${}_{2}$O${}^{+}$+H${}^{+}$ ($72\ifmmode\pm\else\textpm\fi{}4%$), OH${}^{0}$+2H${}^{+}$ ($18\ifmmode\pm\else\textpm\fi{}6%$), and OH${}^{+}$+H${}^{+}$+H${}^{0}$ ($10\ifmmode\pm\else\textpm\fi{}1%$). A kinematic analysis of the H${}_{2}$O${}^{+}$+H${}^{+}$ channel after photoabsorption at 35.56 nm (where only outer valence ionization is possible) showed dissociation into excited states of the water radical ion, where the ${}^{1}{A}_{1}$ state breaks up into the linear $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{A}{\phantom{\rule{0.16em}{0ex}}}^{2}{A}_{1}$ state of H${}_{2}$O${}^{+}$ and the ${}^{2}E$ state decays into the strongly bent $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{B}{\phantom{\rule{0.16em}{0ex}}}^{2}{B}_{2}$ state. Finally, from the ${}^{2}E$ state of H${}_{3}$O${}^{2+}$, dissociation into OH${}^{0}(X{\phantom{\rule{0.16em}{0ex}}}^{2}\ensuremath{\Pi}$)+2H${}^{+}$ was identified to occur with a near linear dissociation geometry.