Proton transfer and hydronium formation in ionized water
Aqueous radiation chemistry emerges through ultrafast proton transfer and ion-radical formation with unexplored energy-redistribution dynamics steering the subsequent reactions. We performed a time-resolved disruptive-probing experiment on pure water dimer, (H$_2$O)$_2$, to disentangle the elementary post-ionization reactions. Through kinetic-energy-resolved ion imaging, we unraveled the dynamics in the (H$_2$O)$_2^+$ ground state: at low-energy ($\sim$0.05~eV) ultrafast proton transfer ($\sim$19~fs) is followed by H$_3$O$^+$+OH fragmentation ($\sim$360~fs). At higher energies, proton transfer becomes hindered ($\sim$60~fs) while the subsequent fragmentation becomes faster ($\sim$210~fs), eventually ($>0.15$~eV) merging into coupled dynamics ($\sim$100~fs). Additionally, we observed H$_2$O)$_2^+$ stabilization proceeding through a Zundel-like structure. These timescales and product energies reveal how ion-radical formation in ionized hydrogen-bonded networks shapes reactivity in aqueous dynamics.