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 time-resolved disruptive probing on pure water dimer, (H$_2$O)$_2$, to disentangle the 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$210 fs), evolving into coupled dynamics ($>0.15$ eV, $\sim$100 fs). Moreover, we observed H$_2$O)$_2^+$ stabilization proceeding through a Zundel-like structure. This reveals how ion-radical formation in ionized hydrogen-bonded networks shapes reactivity in aqueous dynamics.