New Way to Look “Inside” Water’s Microscopic Structure

Water is essential for all chemistry and life, yet understanding how it interacts with dissolved ions—such as sodium and magnesium—has long been a major scientific challenge. Now, an international research team led by Uppsala University has developed a groundbreaking X-ray technique that can directly probe the electronic structure of the solvation shell, the first layer of water molecules surrounding an ion in solution.

The study, published in Nature Communications, reports the first observation of a process called Intermolecular Radiative Decay (IRD) in liquids. In this phenomenon, when an ion in water is ionized by X-rays, an electron from a nearby water molecule fills the ion’s inner-shell vacancy, and the released energy is emitted as an X-ray photon. This emitted photon carries a distinct fingerprint of the ion’s immediate environment—allowing researchers, in effect, to probe the solvation shell “from within.”

“The solvation shell determines how ions behave in water, influencing everything from biological function to corrosion and battery chemistry. Our discovery shows that X-rays can now be used to directly reveal the electronic structure of this critical interfacial region,” says lead author Johan Söderström, Senior Lecturer at the Department of Physics and Astronomy.

Using synchrotron radiation at the MAX IV Laboratory in Lund, Sweden, the team investigated aqueous solutions of sodium and magnesium ions. By analyzing the emitted X-ray photons, they identified distinct spectral signatures originating from neighboring water molecules—clear evidence of the newly discovered IRD process.

Theoretical modeling further confirmed that IRD arises from subtle orbital hybridization between the ion and surrounding water molecules. Remarkably, the process is sensitive only to the first solvation shell, making IRD a uniquely selective probe of local chemical environments in liquids.

“This is the radiative cousin of the well-known Intermolecular Coulombic Decay. But unlike electron-based methods, IRD emits X-rays that can escape from deep within the liquid, enabling us to explore the bulk properties of the solution,” explains senior author Olle Björneholm, Professor at the Department of Physics and Astronomy.

Beyond sodium and magnesium, the researchers have also shown that IRD is observable in other systems, including transition metal ions and anions, suggesting that the phenomenon is general — and potentially transformative for the study of aqueous and biological chemistry.

This discovery paves the way for element- and site-specific studies of solvation structure, chemical bonding, and ultrafast dynamics in liquids using advanced synchrotron and free-electron laser sources.