Tag: tr-XPS

Researchers watch chemistry unfold atom by atom

Researchers watch chemistry unfold atom by atom

Study suggests a new way to follow electronic and vibrational dynamics in real time

Researchers have captured how a molecule redistributes energy after absorbing light, differentiating the roles of individual atoms in the process. They used X-ray flashes from the European XFEL to show that different atoms of the same molecule can reveal entirely different aspects of the process. The study provides clear evidence that excitation by light can enhance an atom’s sensitivity to motion of nearby atoms. The new method for following ultrafast chemical reactions at the atomic scale and in real-time can help understanding photostability in DNA, energy flow in light-harvesting materials, and other fundamental processes driven by light.

The team investigated 3-fluoropyridine, a small ring-shaped molecule. When the molecule absorbs light like a short pulse from an ultraviolet laser, it is promoted into an electronically excited state and rapidly distorts out of its original planar shape. It then passes through a so-called conical intersection: a short-lived but crucial crossing point where movements of electrons and the atoms’ cores become strongly coupled. After this point, the molecule returns to the ground state. At that moment, electronic energy is converted into vibrations. The researchers found that this conversion leaves distinct fingerprints at different atomic sites: the fluorine atom acts as a clean marker of vibrational relaxation, while the nitrogen atom, which is more directly involved in the excitation, reflects an intertwined response of electron redistribution and structural motion. “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.”

To observe this process, the team used time-resolved X-ray photoelectron spectroscopy (tr-XPS) at the Small Quantum Systems instrument (SQS) of European XFEL. An ultraviolet laser pulse first excited the molecules, and a precisely delayed soft X-ray pulse then ionized them by removing deeply bound electrons 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 local chemical environment evolved over the course of just a couple of picoseconds (trillionths of seconds). To interpret the data, the team developed advanced simulations and models.

Read more on the European XFEL website

Image: Experiment on 3-fluoropyridine: a UV pulse (purple beam from the left) excites the molecule, and a delayed X-ray pulse (white beam from the lower right) probes the nitrogen atom (purple sphere). The measurement reveals light-induced charge redistribution (purple cloud) from the perspective of a core-photoelectron from the nitrogen site (concentric rings). The fluorine atom (green sphere) can also be probed and acts mainly as a marker of vibrational dynamics.

Credit: European XFEL/Enrique Sahagun

Chemical shifts help track molecules breaking apart in real time

Chemical shifts help track molecules breaking apart in real time

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