Intermolecular Coulombic decay in liquid water competes with proton transfer and non-adiabatic relaxation.

Despite decades of research, our understanding of radiation damage in aqueous systems remains limited. The recent discovery of Intermolecular Coulombic Decay (ICD) following inner-valence ionization of liquid water raises interesting questions about its efficiency as a major source of low-energy electrons responsible for radiation damage. To investigate, we performed electron-electron coincidence measurements on liquid HO and DO using a monochromatized high-harmonic-generation light source, detecting ICD electrons in coincidence with photoelectrons from the 2a shell. We find that the ICD efficiency γ is below unity in both liquids and that γ(HO)/γ(DO) = 0.86 ± 0.03. Ab initio calculations reveal that ICD competes with proton transfer and non-adiabatic relaxation, which can close the ICD channel. A multi-scale stochastic model incorporating solvent effects reproduces these efficiencies. Our combined experimental and theoretical results suggest that the higher ICD efficiency in DO arises from slower proton transfer and non-adiabatic transitions, highlighting the crucial role of nuclear motion in liquid-phase ICD and advancing the understanding of radiation damage.