Science Programme

FXE science aims to investigate electronic and nuclear dynamic processes in complex chemical systems. We seek to unravel electronic and nuclear dynamic processes during an ongoing light-driven reaction.

Chemistry commences on the femtosecond time scale, when considering nuclear motion (isomerization, making and breaking of bonds). We are interested in the very first steps leading to such drastic changes in reacting molecules. For this reason, we employ new structural tools in the femtosecond time domain to unravel the elementary processes leading to photophysical and hence chemical behaviour. Therefore, it is necessary to understand not only the intramolecular processes themselves, but also guest–host interactions, which equally influence the reaction outcome.

Unravelling complex processes involving electronic and nuclear degrees of freedom, requires precise observables of the ensuing dynamics. The FXE scientific instrument provides a suite of different X-ray tools, embracing spectroscopies and scattering methods, which allow for disentanglement of the simultaneously occurring elementary steps in chemical reactivity.

The Femtosecond X-Ray Experiments group exploits a suite of structural tools available at third-generation synchrotron storage rings and at X-ray free-electron laser (XFEL) sources. We have introduced and implemented several X-ray tools in the picosecond and femtosecond time domains to study not only the mechanistic behavior of reacting molecules, but also the inherent electronic changes that usually drive chemical reactivity in the first place. This research has led to new methods to study chemical activity, also available at the FXE scientific instrument:

 

  1. X-ray absorption spectroscopy tools:
    • XANES (X-ray absorption near edge structure) enables measurement of oxidation state changes, and partially allows extracting molecular symmetries of the reacting species, next to monitoring changes in the LUMOs (lowest unexcited molecular orbitals).

    • EXAFS (extended X-ray absorption fine structure) delivers reliable information about the mocal geometric structure around the investigated atom in the molecular framework.

    • In addition, exploiting an energy-resolving secondary spectrometer for the emitted X-ray fluorescence permits resolution of important features (mainly in the XANES region) significantly below the intrinsic natural line width.

  2. X-ray emission spectroscopy tools:
    • XES (non-resonant X-ray emission) allows extraction of vital internal information: spin-state (via both Kα and Kβ emission spectra), but partially also chemical information (via valence-to-core XES). For this purpose, the FXE scientific instrument hosts two secondary spectrometers: a dispersive type (von Hamos geometry) with limited solid angle acceptance (but with no moving parts) and a spherical analyzer-type (Johann geometry).

    • RXES (resonant X-ray emission spectroscopy) delivers information about

    • XRS (X-ray Raman spectroscopy)

  3. X-ray Diffraction tools:
    • XDS (X-ray diffuse scattering, including WAXS and partially SAXS) can monitor global structural changes in solvated complexes, but, in limited cases, can also deliver some information about the solvation shell around the reacting molecule.

    • XRD (X-ray diffraction) allows for directly recording of structural changes in materials associated with transient processes like phase transitions, coherent photons, charge transfer, etc.