The Science of Radiation and Water: How Proton Transfer Competes with Coulombic Decay

When energetic radiation passes through biological tissue, water molecules in the surroundings can be ionized, triggering cascades of secondary particles. Among these, slow low-energy electrons are known to inflict damage on biomolecules such as DNA. A recent collaboration led by Professor Petr Slavíček from the University of Chemistry and Technology, Prague, together with experimental teams at ETH Zurich, has now provided a refined understanding of one of the key mechanisms by which these slow electrons are generated in liquid water; namely, intermolecular Coulombic decay (ICD); and how that process competes with ultrafast proton transfer and nonadiabatic relaxation. Their results, show that ICD is not a guaranteed outcome after ionization; instead, it must “win a race” against other processes that can redirect the energy before the decay occurs.

Zhang, P., Trester, J., Dubský, J., Kolorenč, P., Slavíček, P., & Wörner, H. J. (2025). Intermolecular Coulombic decay in liquid water competes with proton transfer and non-adiabatic relaxation. Nature Communications, 16(1), 6732. https://doi.org/10.1038/s41467-025-61912-w

This article brings together the main findings from the Prague–Zurich collaboration and related studies, exploring the experimental evidence, isotope effects, modeling framework, and wider implications for radiation chemistry and biomedical physics.

Professor Petr Slavíček from UCT Prague stated,

“Our model predicts all the data that the instruments in these challenging experiments can measure. Therefore, we can also trust it in areas where instruments cannot yet see, and we can explain what happens in a solution after exposure to high-energy radiation.”

When an inner-valence electron is ejected from a water molecule by X-rays or other ionizing radiation, an electronic vacancy is created. One possible way for the system to relax is via ICD: an electron from a neighboring molecule fills that vacancy, and a second electron is expelled, producing a slow electron. Because these slow electrons are especially damaging to DNA and other biomolecules, understanding how efficiently ICD operates in liquid water is important in modeling radiation damage.

In a hydrogen-bonded liquid like water, nuclear motion; particularly proton transfer between molecules; can compete with the ICD pathway. If a proton moves from one water molecule to another quickly, it can alter the local structure such that the ICD channel closes. In other words, the ICD route is not always accessible; proton transfer and nonadiabatic electronic relaxation can divert energy elsewhere before ICD occurs. Previous studies using isolated clusters hinted at this competition, but quantitative understanding in bulk liquid was limited.

The main advance in this study is the combination of high-precision coincidence experiments in both normal water (H₂O) and heavy water (D₂O) with a multiscale stochastic model that integrates quantum mechanical inputs and solvent effects.

The experiment used high-harmonic-generation light to ionize liquid water microjets and measure electron; electron coincidences. This allowed researchers to detect ICD electrons paired with the original photoelectrons from the inner-valence shell. Parallel measurements in H₂O and D₂O made it possible to observe isotope effects, since deuterium atoms move more slowly than hydrogen. The data showed that ICD is less efficient in normal water than in heavy water. This isotope dependence confirmed that the slower nuclear motion in deuterated water gives the electronic ICD process more time to occur.

Because fully quantum simulations of ICD in bulk water are computationally challenging, the researchers built a probabilistic model using quantum-mechanical data from small reference systems like water dimers. These calculations provided potential energy surfaces, proton motion rates, and electronic transition probabilities. The stochastic model then incorporated solvent effects and statistical sampling to simulate how often ICD could “win” against proton transfer or nonadiabatic relaxation. The model accurately reproduced experimental efficiencies and isotope differences, strengthening confidence in its predictive power.

A notable detail is that the stochastic-model work was carried out by Jakub Dubský, then an undergraduate student at UCT Prague, who is now preparing for graduate research at the University of Oxford. Professor Slavíček highlighted that it is rare for an undergraduate student to contribute at such a high level, producing a working model that delivers new knowledge to the field.

The finding that ICD efficiency is below one hundred percent means that ICD cannot be assumed to occur after every ionization event. A significant fraction of cases are dominated by proton transfer or nonadiabatic relaxation. The isotope effect, showing that ICD is comparatively more efficient in heavy water, provides strong evidence that nuclear motion plays a limiting role. The heavier deuteron moves more slowly, giving the ICD process more time to complete before competing mechanisms intervene.

Previous cluster studies had suggested this timescale competition, but the new research provides the first quantitative measurement of it in bulk liquid water. There are still simplifications in the model, such as extrapolating data from small systems and assuming approximate solvent screening, but the agreement between experiment and theory suggests that these approximations capture the main physical picture.

Other theoretical studies have found that proton motion after ionization may oscillate back and forth before either completing a transfer or not, suggesting that the “race” between ICD and proton transfer is more complex than a simple one-way process. Together, these results indicate that ICD and proton transfer are intertwined and form a coupled network of relaxation mechanisms that shape how radiation interacts with water at the molecular level.

This refined understanding of ICD as a conditional process changes how scientists model radiation damage in aqueous environments such as living tissue. Radiation-damage simulations that previously assumed every ionization leads to ICD may overestimate certain pathways. By introducing probabilistic yields that reflect the competition with proton motion, models can more accurately represent the chemistry of ionizing radiation.

In medical contexts like heavy-ion cancer therapy, where radiation interacts strongly with water in cells, the creation of slow electrons through ICD plays a central role in DNA damage. Knowing the limits of this process could help refine treatment strategies, potentially enabling more selective targeting of harmful effects.

The observed isotope dependence also opens theoretical possibilities. Environments with altered nuclear mobility, for instance through isotopic substitution, could change the yield of damaging electrons. While still speculative, this concept might one day inform the design of molecular agents or contrast media that modulate radiation interactions.

The stochastic model also provides predictions for conditions not yet accessible by experiment, such as different temperatures, pressures, or solute environments. Future experiments can test these predictions and feed back into model refinements.

The Prague-Zurich study represents a significant step toward a detailed molecular picture of how slow electrons form in water under radiation. Future research will likely extend these methods to more realistic biological systems containing salts, proteins, or DNA. Simulations that integrate both electronic and nuclear dynamics in larger systems could further clarify how ICD and proton transfer evolve in complex environments.

Time-resolved experiments with finer resolution may soon capture the transient proton motions that determine which pathway wins out. Understanding these sub-femtosecond events will be essential for connecting fundamental radiation physics with real biological effects.

The broader message of this work is that the birth of a single slow electron in water is not a simple, deterministic process. It depends on the delicate timing of electron decay and proton motion, each governed by different aspects of quantum mechanics. This insight reshapes our understanding of radiation chemistry and lays a foundation for more precise modeling of how energy moves through the most common solvent on Earth; water.