Ultrafast proton dissociation in water induced by core ionization: coupled quantum dynamics of electrons and protons from first-principles theory

Understanding the fundamental mechanisms of water radiolysis under high-energy radiation remains a central challenge. Recent advances in attosecond X-ray spectroscopy have enabled the deduction of the long-debated ultrafast proton dissociation in liquid water, occurring within the extremely short lifetime of the oxygen core hole upon core-electron ionization. However, direct evidence remains elusive. We employ the recently developed Ehrenfest dynamics based on a multicomponent density functional theory framework to simulate the coupled quantum dynamics of electrons and protons in liquid water in the core-hole excited state. We find that core ionization of oxygen atoms leads to ultrafast proton transfer within 5–10 femtoseconds, driven by the hydrogen-bond network and dynamically evolving electrostatic fields. The computed non-resonant X-ray emission spectra reproduce the experimentally observed features, providing direct theoretical support for assigning the 1b₁ doublet in liquid water XES to core-hole-induced electron-proton dynamics. This work also demonstrates the time-dependent nuclear-electronic orbital method as a powerful first-principles theoretical framework for studying nuclear-electronic quantum dynamics in condensed-phase systems.