Chemical reactions are often governed by extremely short-lived transition states that only last for fractions of a second. This makes it difficult to follow these lightning-fast rearrangements of bonds and charges. A research team at the European XFEL’s X-ray laser has now achieved this using so-called time-resolved X-ray photoelectron spectroscopy. This enabled them to observe how charges and atomic states in the fluoromethane molecule shift after an electron is ejected and where this happens. The data and additional simulations also revealed which models are suitable for understanding these processes.
Whether the attachment of atoms to a molecule, the disintegration of a compound or the dynamic exchange of binding partners: many chemical reactions are characterized not only by their end products, but also by transition states that only last femtoseconds to picoseconds. They often determine the course of a reaction and also influence the efficiency of the reaction. “It is therefore crucial to know not only the starting molecule and the final fragments, but also the short-lived intermediate states,” says lead author Daniel Rivas from the European XFEL. “These transient species can be highly reactive and represent the actual drivers of chemical changes.” However, observing these intermediate states is not easy because the changes in the molecules occur extremely quickly and on an atomic scale.
A look into the decaying fluoromethane
X-ray lasers such as the European XFEL offer one way to track the rapid changes in chemical reactions. These can generate intense but extremely short X-ray pulses and have high-resolution spectroscopic measuring instruments that can capture energy states in molecules. One of these is the Small Quantum Systems (SQS) instrument, with which Rivas and his team were now able for the first time to observe in detail what happens in a photochemically decaying molecule. For their experiment, the team used the molecule fluoromethane (CH₃F) as a test object. In this case, they first removed an electron from the inner shell of the fluorine or carbon atom using a short laser pulse. As a result of this ionization, bonds and charges in the molecule rearrange. The researchers observed these using time-resolved X-ray photoelectron spectroscopy (tr-XPS). “Photoelectron spectroscopy on inner shells tells us what is happening in a specific atom,” explains Rivas’ colleague Michael Meyer. “By studying carbon and fluorine independently, we can see when different fragments appear and how the charge distribution evolves during dissociation.”
With a total time resolution of about 35 femtoseconds, the team was able to separately track changes at two atom positions, carbon and fluorine, within the same molecule. This revealed that fluoromethane can decompose at very different rates after ionization via different reaction pathways. One of these reaction pathways involves rapid cleavage of the C–F bond, producing a CH₃⁺fragment and a released fluorine atom are created. The second reaction pathway is slower and occurs via cleavage of the C–H bond, producing CH₂F⁺ and a neutral hydrogen atom is formed. The analyzes also showed that chemical shifts are affected by charges that are surprisingly far away, such as when fragments separate and move out of each other’s electric field.
What do you learn from it?
The new findings can help to better understand the behavior of molecules during chemical and photochemical decay reactions – for example in the Earth’s atmosphere or in complex biomolecules. According to the researchers, a better understanding of the resulting highly reactive intermediates is an important basis for being able to better direct and control such photochemical reactions, for example by choosing excitation conditions that favor or suppress certain reaction pathways. And the experiment revealed something else: “We show that a simple and widely used theoretical model, the partial charge model, can be extended to dynamic systems and thus enables the interpretation of the observed signals,” write Rivas and his colleagues. This could greatly simplify the analysis of data from ultrafast XPS in larger, more complex systems.
Source: Daniel Rivas (European XFEL) et al., Physical Review X, doi: 10.1103/y6dt-1sfw