Scientists have captured how energy moves through a molecule immediately after it absorbs light, revealing that different atoms can record distinct stages of an ultrafast chemical reaction.
Using ultrashort X-ray pulses generated at the European XFEL, researchers tracked changes at individual atoms as a molecule released and redistributed absorbed energy. The study shows that light exposure can make one atom more sensitive to the movements of neighboring atoms, providing new insight into molecular energy transfer.
This advanced technique allows scientists to observe ultrafast chemical reactions at the atomic scale and in real time. The findings could help explain how DNA responds to ultraviolet light, how energy moves through light-harvesting materials, and how other fundamental photochemical processes take place.
Tracking Energy Transfer Through a Molecule
The research team examined 3-fluoropyridine, a small ring-shaped molecule that contains nitrogen and fluorine atoms.
When the molecule absorbs energy from a short ultraviolet laser pulse, its electrons move into a higher-energy state. The molecule then rapidly bends away from its usual flat structure.
During this transformation, it passes through a feature known as a conical intersection. This brief but critical point is where the motion of the electrons becomes strongly linked to the movement of the atomic nuclei. Conical intersections are central to many light-triggered chemical reactions because they enable energy to transfer quickly between electronic and structural motion.
After passing through the conical intersection, the molecule returns to its ground state. The remaining electronic energy is converted into vibrations that spread through the molecular structure.
Different Atoms Reveal Different Molecular Changes
The energy conversion generated distinct signals at different atomic sites. The fluorine atom provided a relatively clear view of how the molecule’s vibrations relaxed over time.
The nitrogen atom produced a more complex signal. Because nitrogen was more directly involved in the initial electronic excitation, its response reflected both changes in the electron distribution and changes in the molecule’s 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 an Ultrafast Molecular Transformation
To observe the process, the researchers used time-resolved X-ray photoelectron spectroscopy (tr-XPS) at the Small Quantum Systems instrument (SQS) at the European XFEL.
First, an ultraviolet laser pulse transferred energy to the molecules. A precisely timed soft X-ray pulse then ionized the molecules by removing deeply bound electrons from either nitrogen or fluorine atoms.
The team repeated the experiment while varying the delay between the laser and X-ray pulses. By measuring the energies of the emitted electrons, the researchers reconstructed changes in the chemical environment around each atom over just a few picoseconds, or trillionths of a second.
Advanced computer simulations and theoretical models helped interpret the experimental data and link the observed signals to the molecule’s underlying electronic and structural dynamics.
A New Perspective on Ultrafast Photochemistry
The results demonstrate how the ultrashort, high-brightness X-ray pulses available at the European XFEL can distinguish some of the fastest and most closely connected motions in matter.
Although the experiment examined a relatively simple molecule, the same atomic-scale technique could be used to study more complex systems. Potential applications include functional organic molecules, biomolecular building blocks, and materials designed to absorb, transport, or convert light energy.
“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.”
Source: www.sciencedaily.com