Chemical shifts help track molecules breaking apart in real time
Chemical shifts help track molecules breaking apart in real time
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."
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