A new spectroscopy for ultrafast molecular dynamics: time-resolved shake-down XPS

Life on Earth relies on chemical processes that occur on very fast timescales: photosynthesis in plants, vision in the human eye, and, nowadays, also processes in man-made systems such as solar panels. All of these phenomena are driven by the movement of electrons and atoms which, when exposed to light, reorganize within just a millionth of a billionth of a second (this unit of time is called a femtosecond), a time so short that it completely escapes human perception. Scientists have long been searching for ways to observe these processes “live”.  Understanding how a molecule changes its structure or breaks apart under the influence of light not only reveals the hidden workings of nature, but also paves the way for controlling matter. 

An international team of researchers has published a new study that sheds light on the ultrafast physical-chemical processes occurring when a molecule is excited by ultraviolet radiation. Using ultrashort, highly stable X-ray pulses generated by the FERMI free-electron laser at Elettra-Sincrotrone Trieste, the scientists were able to observe in detail how electrons and atoms within a molecule move and reorganize over the course of just a few femtoseconds. The time-resolved X-ray photoelectron spectroscopy measurements were carried out at the Low Density Matter beamline of FERMI.

The study focuses on carbon disulfide (CS₂), a molecule with a deceptively simple linear structure (S=C=S) yet exhibiting complex dynamics when excited by ultraviolet light—the main reason lies in the presence of the heavy S atoms, which are located sufficiently far down the periodic table to make their electronic structure complicated, and difficult for quantum chemists to accurately model. In particular, certain channels for electron rearrangement are opened by the spin-orbit interaction, which has little practical effects and can be often ignored in calculations for lighter atoms. Through a detailed comparison between experimental data and high-level theoretical calculations (see Figure 1), the researchers detected not only the main fragmentation and primary de-excitation signals of the molecule but also weak "satellite features" associated with specific electronic rearrangements known as shake-down processes.

Figure 1. Experimental time-resolved differential X-ray photoelectron spectrum of CS₂ obtained upon scanning the time delay between the 200 nm pump and 179.9 eV probe, and comparison of the experimental XPS spectra at early (panels c), d) and e)) and late delays (panels a) and b)) with the theoretically calculated ones. Picture adapted from J. Am. Chem. Soc. 147, 36, 32851–32860 (2025).

For the first time, using time-resolved X-ray photoelectron spectroscopy (XPS), satellite signals have been observed in real time during a photochemical reaction. Because their weak signals depend on a concerted rearrangement of the outer electrons (those participating in chemical bonds) and the core electrons (tightly localized around a specific atom) they offer a sensitive and chemically selective probe of both structural and electronic changes that precede molecular dissociation. This new approach, referred to as time-resolved shake-down spectroscopy, provides a powerful and versatile tool with strong potential as a next-generation probe for ultrafast molecular dynamics.

Henry J. Thompson^1,†, Matteo Bonanomi^2,3,†, Jacob Pedersen^4,5 , Oksana Plekan⁶, Nitish Pal⁶, Cesare Grazioli⁷, Kevin C. Prince^8,6, Bruno N. C. Tenorio⁴, Michele Devetta³, Davide Faccialà³, Caterina Vozzi³, Paolo Piseri⁹, Miltcho B. Danailov⁶, Alexander Demidovich⁶, Alexander D. Brynes⁶, Alberto Simoncig⁶, Marco Zangrando^6,7, Marcello Coreno¹⁰, Raimund Feifel¹¹, Richard J. Squibb¹¹, David M. P. Holland¹², Felix Allum¹³, Daniel Rolles¹⁴, Piero Decleva¹⁵, Michael S. Schuurman^16,17, Ruaridh Forbes^13,18, Sonia Coriani^4,19, Carlo Callegari⁶, Russell S. Minns¹ and Michele Di Fraia^7,6.

¹ School of Chemistry and Chemical Engineering, University of Southampton, Southampton, United Kingdom
² Dipartimento di Fisica, Politecnico di Milano, Milano, Italy
³ CNR - Istituto di Fotonica e Nanotecnologie (IFN), Milano, Italy 
⁴ Department of Chemistry, Technical University of Denmark, Kgs. Lyngby, Denmark
⁵ Department of Chemistry, Norwegian University of Science and Technology, Trondheim, Norway
⁶ Elettra - Sincrotrone Trieste S.C.p.A., Basovizza, Trieste, Italy
⁷ CNR - Istituto Officina dei Materiali (IOM), Basovizza, Trieste, Italy 
⁸ Faculty of Mathematics and Physics, Department of Surface and Plasma Science, Charles University, Prague, Czech Republic
⁹ Dipartimento di Fisica “Aldo Pontremoli”, Università degli Studi di Milano, Milano, Italy
¹⁰ CNR - Istituto di Struttura della Materia (ISM), Basovizza, Trieste, Italy 
¹¹ Department of Physics, University of Gothenburg, Gothenburg, Sweden 
¹² Science and Technology Facilities Council (STFC), Daresbury Laboratory, Warrington, UK
¹³ Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California, USA
¹⁴ J.R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, Kansas, USA
¹⁵ Dipartimento di Science Chimiche e Farmaceutiche, Università degli Studi di Trieste, Trieste, Italy
¹⁶ National Research Council Canada, Ottawa, Ontario, Canada 
¹⁷ Department of Chemistry and Biomolecular Sciences, University of Ottawa, Ottawa, Ontario, Canada
¹⁸ Department of Chemistry, University of California, Davis, California, USA 
¹⁹ Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany
^† These authors contributed equally to this work. 

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