The generation of harmful slow electrons in water is a race between intermolecular energy decay and proton transfer

When high-energy radiation interacts with water in living organisms, it creates particles and slow electrons that can later damage critical molecules like DNA. Now Professor Petr Slavíček and his PhD student Jakub Dubský from UCT Prague (University of Chemistry and Technology, Prague) have detailed one of the key mechanisms for creating these slow electrons in water, a process known as intermolecular Coulomb decay (ICD). Their powerful mathematical model successfully explains all the data from the complex laser experiments conducted at ETH Zurich (Hans-Jakob Woerner’s team).

The work that deepens the fundamental understanding of radiation chemistry was published in the magazine Nature Communications.

Detailed knowledge of the process in aqueous solutionscombined with advances in research technologies using high-energy radiation, is transforming the field of radiation chemistry. In the future, these findings could lead to significant changes in various fields, including medicine, especially in the development of more sensitive and controlled applications for devices based on ionizing radiation.

Intermolecular Coulomb decay (ICD) was first demonstrated experimentally in water about 15 years ago, but until recently all experiments were performed on isolated molecules or very small water clusters. New research from the Prague-Zürich collaboration is the first to quantify the competition of ICD with proton transfer and non-adiabatic relaxation in running water and determine the isotopic dependence.

The study shows that once the inner valence electron is ejected from the water molecule by radiation, the ICD process is not 100% efficient. It is in a race with other phenomena, primarily with ultrafast proton transfer between neighboring water molecules and non-adiabatic relaxation. By performing experiments on regular (H₂O) and heavy water (D₂O), researchers have shown that the ICD is more effective in hard water. This isotope effect confirms that the slower motion of the deuterium nuclei gives more time to the electron decay process, providing clear evidence of competition.

“Our model predicts all the data that the instruments in these challenging experiments can measure,” says Professor Slavíček. “So we can also trust it in areas where instruments can’t yet see and we can explain what happens in solution after exposure to high energy radiation.”

The stochastic model is based on inputs from quantum mechanicswhich is usually only possible to calculate for limited systems such as single water molecules or small clusters. These inputs, combined with experimental results, are developed into a probabilistic model that provides a complete picture of ICDs in a realistic environment.

It is interesting that the author of the published stochastic model is Jakub Dubský, who recently graduated from UCT Prague and is about to continue his master’s studies at the University of Oxford.

“It is extraordinary when an undergraduate student submits work at the level of a doctoral candidate, which results in a real, functional product that brings completely new knowledge,” adds Professor Slavíček, praising his student’s contribution.