Ultrafast X-rays capture chemistry unfolding atom by atom

Using rapid X-ray flashes produced at the European XFEL, the researchers followed changes at specific atoms as the molecule released and redistributed the absorbed energy. Their results show that exposure to light can make an atom more responsive to the movement of neighboring atoms.

The technique gives scientists a way to examine extremely fast chemical reactions at the atomic scale and in real time. It could eventually improve understanding of how DNA withstands light exposure, how energy travels through materials designed to harvest light, and how other fundamental light-driven processes occur.

Following Energy Through a Molecule

The researchers studied 3-fluoropyridine, a small ring-shaped molecule containing both nitrogen and fluorine atoms.

When the molecule absorbs energy from a brief ultraviolet laser pulse, its electrons enter a higher energy state. The molecule then quickly bends out of its normally flat structure.

As it changes shape, it passes through what scientists call a conical intersection: a short-lived but crucial crossing point where movements of electrons and the atoms' cores become strongly coupled. These intersections play an important role in many reactions triggered by light because they allow energy to move rapidly between electronic and structural motion.

After crossing this region, the molecule returns to its ground state. The excess electronic energy is then converted into vibrations that travel through the molecular structure.

Different Atoms Reveal Different Changes

The conversion of energy produced distinct signals at different locations within the molecule. The fluorine atom served as a relatively clear indicator of how the molecule's vibrations relaxed over time.

The nitrogen atom told a more complicated story. Because it played a more direct role in the original electronic excitation, its signal reflected both the redistribution of electrons and the molecule's changing structure.

"We can now see that not every atomic site tells the same story in the signals we capture from our X-ray pulses," says Antonio Picón from the Instituto de Ciencia de Materiales de Madrid Consejo Superior de Investigaciones Científicas (ICMM-CSIC), co-author of the study. "Some atoms report where the charge is going, while others reveal how the whole molecule vibrates."

Reconstructing a Picosecond Transformation

To capture the process, the team used time-resolved X-ray photoelectron spectroscopy (tr-XPS) at the Small Quantum Systems instrument (SQS) of European XFEL.

First, an ultraviolet laser pulse delivered energy to the molecules. A carefully timed soft X-ray pulse then ionized them by removing deeply bound electrons from either the nitrogen or fluorine atoms.

The researchers repeated the measurement with the X-ray pulse arriving at many different delays after the initial laser pulse. By recording the energies of the released electrons, they reconstructed how the chemical environment surrounding each atom changed over just a couple of picoseconds (trillionths of seconds).

Advanced computer simulations and theoretical models were then used to interpret the experimental signals and connect them with the underlying electronic and structural changes.

A New View of Ultrafast Photochemistry

The findings demonstrate how the ultrashort, high-brightness X-ray pulses available at European XFEL can separate some of the fastest interconnected motions in matter.

Although the experiment focused on one relatively simple molecule, the same approach could be applied to increasingly complex systems. Possible targets include functional organic molecules, biomolecular building blocks, and materials designed to capture or transfer energy from light.

"This is what European XFEL was built to enable: watching chemical change where it begins, at specific atomic sites and on its natural timescale," says Daniel Rivas, former instrument scientist, now guest scientist at SQS and co-author of the study. "By combining multi-site sensitivity with femtosecond resolution, we are opening a new window on the microscopic mechanisms that govern photochemistry."