Ultrafast X-ray photoelectron spectroscopy at European XFEL offers a new way to watch reactions, atom by atom.
When molecules fall apart, their electric charge doesn’t stay put—it rearranges as bonds stretch and break. An international team of scientists has now tracked these ultrafast changes in the small molecule fluoromethane (CH₃F). It was the first time that the Small Quantum Systems (SQS) instrument at European XFEL could deliver detailed insights into transient states during chemical reactions. These intermediate states, that only exist temporarily while the reaction is ongoing, are often the key drivers of chemistry and therefore crucial to understand. Over the long term, that kind of insight can support progress in areas such as atmospheric science (where sunlight-driven reactions and fragmentation pathways shape air chemistry), as well as the study of complex molecular systems including biomolecules and proteins, where local excitation and charge transfer can trigger structural change.
In the experiment, the researchers first triggered the reaction with an optical laser pulse. Next, they used the X-ray laser pulses that the European XFEL produces, to eject an electron from the core of either the fluorine or the carbon atom in the molecule. They measured the electron’s kinetic energy, which reveals how strongly it was bound inside the atom. That binding energy is extremely sensitive to the local electrical environment, producing so-called “chemical shifts” that act like a fingerprint of the charge distribution surrounding the atom from which the electron has been ejected. With an overall time resolution of about 35 femtoseconds (trillions of times shorter than the blink of an eye), the team could follow changes separately at two atomic sites, carbon and fluorine, inside the same molecule. The method is called time-resolved X-ray photoelectron spectroscopy (tr‑XPS).
“Core-level photoelectron spectroscopy tells us what is happening at a specific atom,” says Michael Meyer, lead scientist at the Small Quantum Systems (SQS) instrument at European XFEL. “By probing carbon and fluorine independently, we can see when different fragments appear and how the charge distribution evolves during dissociation.
Read more on the European XFEL website
Image: Illustration of the pump–probe experiment on fluoromethane (CH₃F): Shortly after an ultrashort optical laser pulse (red) has ionized the molecule and triggered bond breaking, a femtosecond X-ray pulse (blue/white) ejects a core electron (green clouds) from the fluorine atom (green ball). By measuring the electron’s kinetic energy, the experiment tracks time-dependent ‘chemical shifts’ that reveal how the local electronic environment changes as the molecule dissociates – in this case the departure of a hydrogen atom (white ball).
Credit: Illustration: European XFEL