Scientists observe molecular energy redistribution atom-by-atom using X-ray pulses

A team of researchers has done something that seemed impossible until very recently: they watched a single molecule absorb light and then tracked, atom by atom, how it redistributed the energy it had just captured. The molecule in question was 3-fluoropyridine, a small ring-shaped structure made of carbon, hydrogen, nitrogen, and fluorine atoms. When hit with an ultraviolet laser pulse, the molecule jumped into an excited state and twisted out of its normal flat shape. What happened next was the real story—and it unfolded in picoseconds, trillionths of a second.

The journey the molecule took after that initial excitation was intricate. It passed through what physicists call a conical intersection, a fleeting but critical moment where the movement of electrons and the movement of atomic nuclei become tightly linked. Once through that crossing point, the molecule settled back down to its ground state. But the energy it had absorbed didn't simply vanish. Instead, it transformed into vibrations—the atoms began to shake and move in patterns that carried information about what had just happened.

What made this observation remarkable was not just that the researchers could see the process, but that they could see it differently at different atomic sites. The fluorine atom, it turned out, acted like a clean signal of vibrational relaxation—a straightforward readout of how the molecule was vibrating after the energy conversion. The nitrogen atom told a more complicated story. Because nitrogen had been more directly involved in the initial excitation, its signal reflected something messier and more intertwined: both the redistribution of electrons and the structural motion of the molecule happening at once. "We can now see that not every atomic site tells the same story in the signals we capture from our X-ray pulses," explained Antonio Picón, a researcher at the Instituto de Ciencia de Materiales de Madrid. "Some atoms report where the charge is going, while others reveal how the whole molecule vibrates."

To pull off this feat, the team used an instrument called the Small Quantum Systems beamline at the European XFEL, a facility in Germany that produces extraordinarily bright and brief pulses of X-rays. The experimental sequence was precise: first, an ultraviolet laser excited the molecules. Then, at carefully chosen moments afterward, a soft X-ray pulse ionized the atoms by knocking out their deeply bound electrons. By measuring the energy of those ejected electrons across many different time delays, the researchers could reconstruct how the chemical environment around each atom changed over the course of just a couple of picoseconds. The data alone would have been meaningless without interpretation, so the team built advanced simulations and computational models to make sense of what they were seeing.

The implications reach far beyond this one molecule. The technique demonstrates that European XFEL's ultrashort, high-brightness X-ray pulses can untangle the fastest and most coupled motions in matter—the kinds of processes that happen so quickly and involve so many simultaneous changes that they have always been difficult to study. Daniel Rivas, a scientist at the facility, framed it this way: "This is what European XFEL was built to enable: watching chemical change where it begins, at specific atomic sites and on its natural timescale. By combining multi-site sensitivity with femtosecond resolution, we are opening a new window on the microscopic mechanisms that govern photochemistry."

The approach is general enough to be applied to far more complex systems. Researchers can now begin to investigate how light triggers structural change in functional organic molecules, in the building blocks of biomolecules, and in materials designed to capture and convert energy. Each of these systems involves photochemistry—the chemistry that light sets in motion—and understanding it at the atomic level, in real time, could reshape how scientists design new materials and drugs. The window that has just opened is only beginning to show what lies beyond it.