New method for "capturing" changes in chemical bonds due to electronic dynamics

Schematic of two XFELs pulses to study the chemical environment around the carbo
Schematic of two XFELs pulses to study the chemical environment around the carbon atom in real time / A. Picón

An international collaboration involving the Autonomous University of Madrid (UAM) has succeeded in bringing X-ray photoelectron spectroscopy to the femtosecond time scale. This advance, published in Nature Communications, will not only make it possible to investigate the effects on chemical bonds due to the movements of atomic nuclei, but also those caused by electronic dynamics.

X-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis, is a central tool for studying the local chemical environment at the atomic level.

Taking advantage of the atomic selectivity of X-rays, XPS spectroscopy has become one of the most widely used techniques for surface analysis. Most XPS measurements have had limited temporal resolution due to the characteristics of the X-ray source, where synchrotrons pushed the XPS limits into the tens of picosecond (10-12 s) regime.

Now, an international collaboration with groups from the Universidad Autónoma de Madrid (UAM), École Polytechnique Fédérale de Lausanne (Switzerland), Paul Scherrer Institute (Switzerland), Argonne National Laboratory (USA) and Stanford University (USA), has succeeded in showing chemical shifts in an ultrafast XPS scheme of only a few femtoseconds (10-15 s).

Such temporal resolution allows XPS spectroscopy not only to investigate the motions of the nuclei, but also to capture the electronic dynamics on their natural time scale.

"We will now be able to observe changes in the chemical environment just after light excitation, which opens up a new panorama of applications, such as in the understanding of photocatalytic complexes, photochemical processes and the photoresistance of biologically relevant molecules," the authors state.

Ultra-fast XPS scheme

Kai Siegbahn received the Nobel Prize in Physics in 1981 for his pioneering contribution and understanding of chemical energy shifts in XPS experiments. The advent of X-ray free electron lasers (XFELs) has opened the door to extend XPS studies to the femtosecond time domain and thus explore out-of-equilibrium dynamics with a time resolution of only a few femtoseconds.

Extending XPS to the femtosecond regime is not straightforward and requires the development of a new methodology to extract information in an evolving system. Ultrafast chemical shifts of energy out of equilibrium are a completely unexplored field, and understanding them is critical to correlate with real-time changes of local chemical bonds.

In the new scheme, published in Nature Communications, the researchers employed two consecutive femtosecond X-ray pulses with different frequencies. "The first X-ray pulse is resonantly tuned to locally excite the oxygen (O) of the carbon monoxide (CO) molecule, while the second X-ray pulse follows the local chemical environment of the carbon (C) side after a few femtoseconds," they explain.

"Chemical energy shifts," the researchers continue, "inform about the shielding effect when the O atom is excited, and also about the Auger decay triggered by electron interactions and the subsequent fragmentation of the molecule into two separate atoms.

The experimental and theoretical method used in this paper paves the way for future experiments on XFELs to better understand ultrafast chemical shifts, thus being closer to the dream of resolving local chemical bond shifts in real time.

Bibliographic reference:

Al-Haddad, A., Oberli, S., González-Vázquez, J., Bucher, M., Doumy, G., Ho, P., Krzywinski, J., Lane, T.J., Lutman, A., Marinelli, A., Maxwell, T.J., Moeller, S., Pratt, S.T., Ray, D., Shepard, R., Southworth, S.H., Vazquez-Mayagoitia, A., Walter, P., Young, L., Picón, A., Bostedt, C. 2022. Observation of site-selective chemical bond changes via ultrafast chemical shifts. Nature Communications 13, 7170.

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