X-ray spectroscopy study maps ultrafast charge delocalization in aqueous environments

22 Oct 2024, 18:52 by University of Chemistry and Technology Prague

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2D electron signal maps showing the electron kinetic energy as a function of photon energy, scanned across the cation 1s ionization thresholds, for Na⁺, Mg²⁺, and Al³⁺ ions in water. Credit: University of Chemistry and Technology Prague

The movement of electron density is a subject of interest for chemists worldwide, as substances interact through electrons. These processes are ultrafast and traditionally require time-resolved experiments at the attosecond level to study and describe. However, there are alternative methods.

Research teams led by Olle Björneholm from Uppsala University, in cooperation with the theory group led by Eva Muchová from the University of Chemistry and Technology, Prague, have discovered that this ultrafast charge delocalization can be examined using a simpler method—X-ray spectroscopy. They are also the first in the world to describe the time it takes for a hydrated electron to form and separate from sodium, magnesium, and aluminum ions in water. Their findings are currently featured in the journal Nature Communications.

"What happens to an electron after it leaves an ion in an aqueous environment is a fascinating scientific topic addressed in many papers. However, what was missing from the puzzle was the timescale for the initial steps—the formation of the hydrated electron and the very first moments of its detachment from the ion," says Muchová.

"Everything happens so quickly that, with the current method of time-resolved experiments, we would need attosecond laser pulses, which are not easily accessible. Moreover, we still don't know if they would even work correctly. So, we approached it differently."

Sketch of the discussed processes. Credit: University of Chemistry and Technology Prague

By "differently," she means observing the described process not through time resolution but by resolving it in energy as 2D maps. These maps can be imagined as photographs taken with a long exposure time, where motion can be observed even though the moving object is too fast to be sharply captured. By analyzing the signal, they were able to reconstruct how fast the electron delocalizes from the ions.

The experiment, carried out at the DESY synchrotron in Hamburg, demonstrated that the time it takes for water to "carry away" the electron from the ion depends on the electric charge strength of the ion and, consequently, the arrangement of the surrounding water molecules. In this analogy, sodium acts like a carefree parent who lets their child swim, aluminum behaves as a strict protector, and magnesium falls somewhere in between.

Why is this discovery important? Understanding another part of the complex and diverse process of electron transfer using more accessible techniques could benefit various fields in the future, as voltage transfer occurs in biological organisms, photocatalysis, and even in the solid phase.

More information: E. Muchová et al, Attosecond formation of charge-transfer-to-solvent states of aqueous ions probed using the core-hole-clock technique, Nature Communications (2024). DOI: 10.1038/s41467-024-52740-5

Journal information: Nature Communications

Provided by University of Chemistry and Technology Prague