Atoms Don’t Sit Still: Scientists Catch Them Roaming Before X-Ray Damage Strikes

Researchers have uncovered how atoms subtly rearrange themselves for up to a trillionth of a second before releasing low-energy electrons after X-ray excitation.

Together with collaborators from around the world, scientists from the Molecular Physics Department at the Fritz Haber Institute have uncovered how atoms change their arrangement before emitting low energy electrons during a decay triggered by X-ray radiation. For the first time, the team was able to resolve the timing of this process in detail, offering new insight into mechanisms that contribute to radiation damage.

Radiation-induced decay processes

When high-energy radiation, such as X-rays, interacts with matter, it can leave atoms and molecules in excited, unstable states. Those excited states typically relax through decay processes that can break chemical bonds and disrupt the biomolecules that cells rely on. Because many different decay routes can follow the initial hit, identifying the specific steps and their timing is a key part of understanding why radiation can be so damaging, and how that damage might be limited.

In this study, the team focused on electron-transfer-mediated decay (ETMD), a mechanism that is especially important in radiation chemistry and biological damage research because it can produce low-energy electrons through interactions between neighboring atoms. In ETMD, an irradiated atom relaxes by taking an electron from a nearby partner, and the energy released in that transfer then ionizes another neighbor. In other words, the decay is non local, with multiple atoms sharing the burden of stabilizing the system.

Using a carefully chosen model system, the team was able to follow how atoms move and reorganize themselves before this unusual electronic decay takes place. Their results represent the most detailed view so far of ETMD in both real space and real time.

Sophisticated combination of experiment and theory

To achieve this, the international collaboration studied a simple system made up of one neon atom weakly bound to two krypton atoms (NeKr2 trimer). After the neon core was ionized with soft X-rays, the scientists tracked the system for up to one picosecond, a very long time on the atomic scale, before it ultimately decayed by transferring an electron between neighboring atoms and releasing a low-energy electron.

The measurements were carried out using an advanced COLTRIMS reaction microscope at the synchrotron facilities BESSY II (Berlin) and PETRA III (Hamburg). This allowed the researchers to reconstruct the exact arrangement of the atoms at the moment the decay occurred. To fully understand the experimental data, they also performed fully dimensional ab initio simulations, following thousands of possible atomic motion pathways and calculating the likelihood of decay along each one.

Taking a movie of the non-local electronic decay

What they discovered was striking: the atoms do not remain frozen in their initial configuration. Instead, they undergo pronounced roaming-like motion, continuously reshaping the molecular geometry and strongly influencing when and how the decay occurs.

“We can literally watch how the atoms move before the decay happens,” says Florian Trinter, one of the lead authors. “The decay is not just an electronic process – it is steered by nuclear motion in a very direct and intuitive way.”

The results reveal that ETMD does not happen from a single “preferred” structure. Instead, different molecular geometries dominate at different times: At early times, the decay occurs near the ground-state geometry, while at intermediate times one krypton atom approaches the neon atom closely and the second drifts farther away – an optimal setup for electron donation and long-range energy transfer. At later times, the system explores almost linear and highly distorted configurations, reflecting a pendular, roaming-like motion of the atoms. This dynamic reshaping leads to strongly time-dependent decay rates, varying by nearly an order of magnitude depending on geometry.

“The atoms explore large regions of configuration space before the decay finally takes place,” explains Till Jahnke, senior author of the study. “This shows that nuclear motion is not a minor correction – it fundamentally controls the efficiency of non-local electronic decay.”

Why It Matters

ETMD has attracted increasing attention because it efficiently produces low-energy electrons, which are known to cause chemical damage in liquids and biological matter. Understanding how ETMD depends on molecular structure and motion is therefore crucial for modeling radiation damage in water and biomolecular environments, as well as for interpreting ultrafast X-ray experiments. Moreover, the current findings are very helpful for developing multiscale theoretical approaches that embed accurate decay rates into large, complex systems.

By providing a detailed benchmark for the smallest system that supports ETMD involving three atoms, the present study lays the groundwork for extending these ideas to liquids, solvated ions, and biological environments.

“This work shows how non-local electronic decay can be used as a powerful probe of molecular motion,” the authors conclude. “It opens the door to imaging ultrafast dynamics in weakly bound matter with unprecedented detail.”

Reference: “Tracking the Complex Dynamics of Electron-Transfer-Mediated Decay in Real Space and Time” by Florian Trinter, Jaroslav Hofierka, Jonas Rist, Max Kircher, Miriam Weller, Niklas Melzer, Dimitrios Tsitsonis, Angelina Geyer, Jan Kruse, Gregor Kastirke, Joshua B. Williams, Tsveta Miteva, Reinhard Dörner, Markus S. Schöffler, Maksim Kunitski, Nicolas Sisourat, Lorenz S. Cederbaum and Till Jahnke, 22 January 2026, Journal of the American Chemical Society.
DOI: 10.1021/jacs.5c15510

Funding: Deutsche Forschungsgemeinschaft, European Research Council

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