Researchers observe chemical changes atom by atom

Researchers have observed how a molecule redistributes energy after the absorption of light, and have been able to distinguish the roles of individual atoms in this process. Using X-ray flashes from the European XFEL, they showed that different atoms of the same molecule can reflect completely different aspects of the process. The study provides clear evidence that excitation by light can increase an atom's sensitivity to the movement of adjacent atoms. The new method of tracking ultra-fast chemical interactions at the atomic level and in real time can contribute to understanding the photosensitivity of DNA, energy fluxes in light-absorbing materials, and other fundamental processes excited by light.

The team studied 3-fluoropyridine, a small annular molecule. to instance, when the molecule absorbs light from a short pulse of a UV laser, it is placed in an electronically excited state and rapidly deforms from its original planar shape. It then passes through what is known as a conical crossing point: a short-lived however crucial state where the movements of the electrons and the atomic nuclei are strongly coupled. After this point, the molecule returns to the ground state. At this moment, electronic energy is converted into vibrations. The researchers found that this conversion leaves clear traces at different atomic locations: the fluorine atom serves as a clear marker to vibration relaxation, while the nitrogen atom, which is greater immediately involved in excitation, reflects an intertwined interaction of electron redistribution and structural motion. “We can now see that not every atomic position in the signals we capture from our X-ray pulses tells the same story,” says Antonio Picón of the Instituto de Ciencia de Materiales de Madrid Consejo Superior de Investigaciones Científicas (ICMM-CSIC), co-author of the study. “Some atoms indicate where the charge is headed, while others reveal how the whole molecule oscillates.”

To observe this process, the team utilized time-resolved X-ray photoelectron spectroscopy (tr-XPS) at the Small Quantum Systems (SQS) experiment station at the European XFEL. An ultraviolet laser pulse initially excited the molecules. Subsequently-with a precisely adjusted delay-a pulse in the area of the soft X-ray light ionized the molecules by removing bound electrons from deep layers from either the nitrogen or fluorine atoms. By measuring the energy of these emitted electrons at many different time delays, the scientists reconstructed how the regional chemical ecological stability evolved over the course of only a few picoseconds (trillionths of a second). To interpret the data, the team developed cutting-edge simulations and models.

The research confirms the ability of the European XFEL's ultrashort, high-intensity X-ray pulses to untangle the fastest coupled movements into matter. Beyond this particular molecule, the approach can be applied generally to examine how light triggers structural changes, with the possible to study ever greater complex systems-from functional organic molecules to biomolecular building blocks and energy materials. “This is exactly what the European XFEL was built to: to observe chemical changes where they begin-at certain atomic sites and on their natural time scale,” says Daniel Rivas, a former instrument scientist, now a Visiting Scientist at SQS and co-author of the study. “By combining sensitivity to multiple atom positions with a resolution in the femtosecond range, we open a new window to the microscopic mechanisms underlying photochemistry.”