Cluster-Size and Time-Resolved XUV Photoelectron Spectroscopy of Isolated Photocatalytic Water Splitting Systems

• Applicant: Dr. Sebastian Hartweg
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas ; Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 556292160

The photocatalytic splitting of water for the production of molecular hydrogen and oxygen on industrial scales, following the example of photosynthesis, is an elusive technological goal. The relevance and importance of this goal seem to increase recently, as societies around the globe struggle to find new sources of renewable energy. Part of the problem obstructing progress toward this goal is the surprising complexity of the natural photosynthetic apparatus, which limits fundamental insights in the working mechanisms and processes from experimental studies. Even simpler model systems of greatly reduced complexity could so far at most be studied in the liquid solution phase, thus limiting the range of applicable experimental methods as well as the details of the reaction steps resolvable in their results. A precise molecular level understanding of the involved electron and proton transfer reactions is however crucial for a mechanistically-guided search for alternative, technologically usable water splitting systems. This project will use simple model systems of minimal size that show the ability to split water molecules after optical excitation, to obtain deep physical insights into the involved processes. These model systems, in the form of clusters of a molecular photocatalyst (i.e. a chromophore molecule) with water molecules, can be isolated in the gas phase or in superfluid helium nanodroplets, thus allowing their study with the full set of photoionization techniques, free from distorting effects of the environment. Specifically, we will perform cluster-size and time-resolved photoelectron spectroscopy on these systems, allowing the direct observation of all intermediate steps leading to the cleavage of the molecular bond in the water molecule. For these studies we will combine a high-repetition rate optical femtosecond pump-probe setup, consisting of a tuneable laser and a table-top source of monochromatized XUV pulses based on high harmonic generation with a photoelectron photoion coincidence spectrometer. While, all the experimental techniques proposed in this project have been demonstrated in the past, their combination and application to hydrogen-bonded clusters has not been reported and creates a new experimental technique, providing unprecedented mechanistic understanding of the processes contributing to the photocatalytic splitting of water.

• DFG Programme: Emmy Noether Independent Junior Research Groups
• Major Instrumentation: Delay-Line Detektor 75 mm ; Multipass cell pulse compression ; Optical parametric amplifier ; Yb-Laser-System
• Instrumentation Group: 5700 Festkörper-Laser ; 5770 Lichtmodulatoren, Elektrooptik, Magnetooptik ; 5800 Photodetektoren, -zellen, -widerstände für UV-VIS