Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - EDAX (Beating Complexity through Selectivity:Excited state Dynamics from Anti-Stokes and non-linear resonant inelastic X-ray scattering)

Teaser

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...

Summary

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.

Work performed

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

Final results

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.