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Content archived on 2024-06-18

Visualizing Molecular Change

Final Report Summary - VISCHEM (Visualizing Molecular Change)

A table-top setup for ultrafast studies of molecular structure dynamics has been constructed. It is based on a femtosecond laser plasma source for generation of femtosecond X-ray pulses and a superconducting transition edge sensor (TES) detector for energy resolved X-ray detection. Visible pump-X-ray probe measurements can be performed with this setup to monitor molecule structure dynamics. At present, static X-ray absorption and emission spectra of several different transition metal complexes of interest for photocatalysis and solar energy conversion have been measured. The first successful time resolved pump-probe measurements have been performed and from these measurements it was concluded that for implementing the full time resolved capacity of the setup the energy resolution and count rate of the TES detector have to be increased. These developments are underway and predicted to be completed within approx. 6 months.

Synchrotron X-ray picosecond pulses have also been used in order to explore light induced changes of electronic and atomic structure of photocatalytic molecules. By combining X-ray absorption and diffuse scattering measurements information on local as well as global structural changes was obtained. In particular, a bimetallic Ru=Co photocatalysis model system was investigated and we could demonstrate the power of the X-ray techniques. Light absorption by the Ru-moiety is believed to lead to electron transfer to the Co-center, where catalysis can occur (hydrogen gas generation). However, the Co-center is optically dark and conventional optical spectroscopy is therefore not sensitive to the electron transfer. Time resolved X-ray measurements could unambiguously demonstrate the reduction of the Co-center and associated structural changes (changes of Co-N bond lengths).

During the last approx. 2 years X-ray free electron lasers (XFELs) have become available for users. We have used the two facilities available, LCLS in Stanford and Sacla in Japan, to study complex light induced processes in photocatalytic model systems. On the Ru=Co complex discussed above we could resolve the complete sequence of electron transfer, spin change and associated change of structure. These are the first measurements of their kind for such a complex system and processes, demonstrating the potential of X-ray methods to monitor and disentangle complex changes of electronic and atomic structure of large molecular systems. This will be powerful future techniques for understanding function of complex molecular and material systems.