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

Reporting period: 2019-01-01 to 2020-06-30

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.
In the second reporting period (Month 19-30) following the first period (Month 0-18) we have proceeded for the three activities described within EDAX considerably and are within schedule or for some fields even ahead of the anticipated scientific progress. 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.”
After the physical core-principle of Anti-Stokes RIXS has been put on a solid experimental and interpretational footing within the first reporting period for Fe(CO)5 undergoing ligand substitution, we have established in the second period that the inversion symmetric molecular system of Fe(CN)6 also has strong symmetry related RIXS fingerprints of the optically excited state, which we published. Based on these signatures detailed work on charge separation within the molecule and the injection of charge in the i.e. aqueous solvent is established and prepared for publication. This has become scientific focus of PhD student Jay. The investigation of excited state proton transfer is central to PhD student Eckert, where also clearly distinct excited state signatures in RIXS could be established and published and now deep insight to highly relevant issues like photoprotection have been observed and published. Based on this excellent progress PhD student Eckert intents to submit his thesis in September 2018 and continue on a Postdoctoral level.

For the solid state detection of excited state dynamics in the metal insulator phase transition of Fe3O4 in period one we had first indications from two-colour X-ray/X-ray pump probe at FLASH and could in period two finish now at beamtime at LCLS remarkable experimental data on the decoupling of electronic excitation and lattice parameters by our proposed path of ultrafast X-ray/X-ray pump probe in Bragg condition. The resulting first manuscript is with reviewers right now constituting an important step towards multi-centre dynamics shown with Fe3O4. As previously reported in phase one, our characterization efforts of Bragg enhancement of RIXS signals in the phase transition region of Fe3O4 due to critical fluctuation, as previously predicted theoretically, at ESRF and BESSY II established unfortunatlly for Fe3O4 no enhancement stronger than the elastic contribution within the region of critical fluctuation behaviour. This could be due to a minority channel caused by the degree of itinerancy/localization within the particular system. Otherwise it could be of fundamental nature, since the statistical phase shift for inelastic scattering could always outcompete any coherent superposition. As stated at the end of period one, we therefore decided to widen in the second period the base of phase transition material in a dedicated push from solely Fe3O4 to a set of transition metal oxides and sulphides (dichalcogenides), such as TaS2, MoS2, V2O5, which we characterized in period two and are still currently characterizing with regard to multi-centre charge transfer dynamics and the occurrence of transient excited states. We have established relationship to the Uppsala University nano-lab for sample growth and significant progress in the characterization of Dichalcogenide phase transition behaviour has been made. One manuscript on the interlayer coupling in the commensurability phase transition of TaS2 is under review and manuscripts on MoS2 are under preparation. This work is conducted by PostDoc Sorgenfrei and Neppl with PhD student Kühn, who is defending his thesis in Fall 2018. Thus, significant progress has now been achieved both with the “weak” signal of Fe3O4 in multicolour X-ray/X-ray pump probe as a proof of principle and the characterization of a larger set of phase transition materials, in particular from the family of the dichalcogenides.

• “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.”
In period one, The X-ray optical element to implement Anti-Stokes RIXS at European XFEL with transform limited pulses has been simulated, specified and ordered through European XFEL following our science based requests. In period two we realized that the higher optical quality of the high resolution X-ray optical element also translates to enhanced stability requirements in the XFEL base line mechanical layout. To ensure appropriate specification I was able to secure the support by already retired X-ray optics and instrumentation specialist Dr. Friedmar Senf to support the difficult technical specifications part with his advise and experience in discussion with the commercial company building for European XFEL the turn key base line spectrometer that is upgraded to the high resolution capability within EDAX. He will also play a crucial role, when the EDAX specified optical performance is to be established in commissioning. We aim to summarize our insights on the high resolution X-ray optics and conceptual high stability instrument design in a technical publication once operation is reached.
• “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.
In period two the highly successful approach 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.
Publications on solution situations as well as crystalline condensed phases are in preparation. 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, once operable, is possible with well characterized systems.

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. A set of publications is in preparation.
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.