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Beating Complexity through Selectivity: Excited state Dynamics from Anti-Stokes and non-linear resonant inelastic X-ray scattering

Periodic Reporting for period 4 - EDAX (Beating Complexity through Selectivity:Excited state Dynamics from Anti-Stokes and non-linear resonant inelastic X-ray scattering)

Reporting period: 2020-07-01 to 2021-12-31

How can we govern rate and selectivity in chemistry and catalysis? How can we harvest, convert and store energy efficiently? How can switching and phase transitions become more energy efficient for information technologies? These issues all reflect on the societal need to not only describe the properties of materials and chemical processes, but move towards the governing principles of functionality in order to gain predictive power and eventually exert control. Common to this endeavor is, that the excited states of matter and their temporal evolution need characterization down to the level of individual atoms. Here the selectivity of soft X-rays has a proven track record of picking up the elemental composition, the chemical as well as magnetic state of matter to determine its properties. However, gaining selectivity to the low concentration of excited states within the bulk of a material or a solution environment remains a challenge. In addition, the potential energy surface around selected atoms becomes accessible by sub-natural linewidth resonant inelastic X-ray scattering. EDAX addresses these issues on a select set of photochemical reactions in solution and phase transition materials, where “excited state dynamics with Anti-Stokes resonant inelastic X-ray scattering” gives a novel avenue to follow excitations and their evolution selectively. We conduct proof of principle experiments at Free Electron Lasers and Synchrotrons and in a second step advise and ensure implementation of optimum experimental conditions for excited state dynamics at the Synchrotron and European XFEL. The conceptual insights we gain on efficient conversion and chemical processes are the basis to replace the often rare and/or toxic active sites with abundant and harmless elements. To this end EDAX has brought excited state detection of active sites to an unprecedented level.
In the 2nd period we have concluded the three activities described within EDAX and have been able to broaden even the scope of systems significantly, as already indicated in the previous reports. Thanks to the excellent scientists and PhD students scientific staff within EDAX. These are:
- “Proof-of-principle of Anti-Stokes RIXS as a superior probe of excited state dynamics and soft X-ray 4 wave mixing for multi-centre dynamics at an atomic scale at the existing brilliant Free-Electron Laser facilities (FLASH, Germany; FERMI, Italy; LCLS, USA) with a compact transportable set-up.”
- “Establish optimum conditions with transform limited pulses and a dedicated set-up at the European XFEL with unprecedented brilliance for Anti-Stokes RIXS and resonant soft X-ray 4 wave mixing.”
- “Derive complementary potential energy surfaces and excitations from sub-natural linewidth RIXS at the Swiss Light Source and €RIXS at the European Synchrotron Radiation Facility. At the Synchrotron BESSY Germany static RIXS, kinetics and driven phases.

The highly successful approach was to map out potential energy surfaces along selected coordinates, which we published in a series of publications. In particular for water as highly relevant solvent of our excited state molecules significant progress has been made. In the final period, we have been able to significantly enhance our understanding of the compatibility of continuous distribution models in liquid water at ambient conditions, highly relevant as the solvent of our photochemically active solutes.
Publications on solution situations as well as crystalline condensed phases have been continuously put forward and even after the project end we will continue to publish material based on the potential mapping approach. Having established this solid basis on the potential mapping approach for the static molecules/materials gives us great certainty, that the combination of potential energy surface mapping for excited states at European XFEL is a powerful excited state dynamcis tool. Computational science colleagues and the EDAX PI have written an invited “Review of Modern Physics” on “Dynamics of resonant X-ray and Auger scattering”, making many of these insights and approaches of EDAX available to a larger scientific community.

At BESSY, Germany we have both characterized with low resolution Fe based complexes, Proton and charge transfer systems with regard to their electronic structure, their long-time kinetics and their response to chemical variation such as pH dependence. In particular strongly improved experimentation due to the gained experience of the involved scientists with picosecond temporal resolution allows to characterize excited state behaviour of molecular and solids highly efficiently with soft X-ray spectroscopic approaches, in particular RIXS. Based on the experimental count rates achieved, we try to establish, if even at the Synchrotron radiation source Anti-Stokes RIXS could be possible, which would open the approach significantly independent of FEL sources. We have termed this campaign EDAX@VSR at the BESSY Synchrotron radiation source. With R. Büchner a PhD student significantly advanced our understanding beyond the Fe-based complexes towards photoexcitation in Porphyrins, where in particular the established Gouterman-model has been reformulated based on the balance of local central charge and covalent ligand rearrangement. R. Büchner handed in the thesis at the EDAX end and defends in spring 2022.
The establishment of Resonant inelastic X-ray scattering as a background free probe to excited state dynamics on the level of individual excited atoms is based on the establishment of the X-ray spectroscopic fingerprints. This has recently been achieved and is the foundation to determine dynamic pathways for ligand exchange, charge separation and proton transfer. Beeing embedded in solution environment has made our approach of potential energy surface mapping around selected atoms with sub -natural linewidth resonant inelastic X-ray scattering equally groundbreaking. We thus follow excitation pathways in chemical and phase transitions materials with innovative X-ray spectroscopy down to the level of atoms and derive governing principles of functionality in close collaboration with first principles theoretical modelling.
A laser system for picosecond transient X-ray absorption measurements is in the construction phase.