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, which featured Zhong Yin, an associate professor at Tohoku University's SRIS, 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," says lead author Johan Söderström. "Our discovery shows that X-rays can now be used to directly reveal the electronic structure of this critical interfacial region."
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," explains senior author Olle Björneholm. "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."
"ICD is a well-known non-local decay channel and a prominent source of slow electrons, which are particularly interesting because of their ability to induce DNA strand breaks. We were wondering whether an analogous process might exist for photons," says Zhong Yin.
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.
- Publication Details:
Title: New X-ray Technique Reveals Hidden Structure around Ions in Water
Authors: Johan Söderström, Lucas M. Cornetta, Victor Ekholm, Vincenco Carravetta, Arnaldo Naves de Brito, Ricardo Marinho, Marcus Agåker, Takashi Tokushima, Conny Såthe, Anirudha Ghost, Dana Bloß, Andreas Hans, Florian Trinter, Iyas Ismail, Debora Vasconcelos, Joel Pinheiro, Yi-Ping Chang, Manuel Harder, Zhong Yin, Joseph Nordgren, Gunnar Öhrwall, Hans Ågren, Jan-Erik Rubensson and Olle Björneholm
Journal: Nature Communications
