Probing the electronic structure of manganese complexes with X-ray absorption and emission spectroscopy using an X-ray free-electron laser

Speaker: Thomas Fransson, Stanford PULSE Institute, SLAC National Accelerator Laboratory
Date & Time: Mon, 2017-10-23 11:00 - 12:00
Location: AMPEL 311
Local Contact: George Sawatzky
Intended Audience:

Understanding the structure and dynamics of transition metal complexes has long been one of the grand challenges of inorganic chemistry, with numerous applications in, e.g., the development of energy-efficient catalysts, the investigation of metalloenzymes and improvement of pharmaceuticals, and artificial photosynthesis. In this talk I will present the development and application of X-ray spectroscopy methods for investigating such complexes, with emphasis on those containing manganese – a long-standing goal of this research is a molecular understanding of the splitting of water into atmospheric dioxygen in photosynthesis, which take place in the oxygen-evolving center (OEC) of photosystem II.

However, in order to obtain time-resolved measurements of samples under ambient conditions, free from damages induced by the X-ray radiation, very intense and short photon pulses are needed. As such, the synchrotron facilities often employed in X-ray sciences are insufficient, and we instead utilize modern X-ray free-electron laser (XFEL) facilities. I will thus describe the basic principles of such facilities, contrasting this to more standard X-ray sources and describe some of the challenges and opportunities provided.

In joint efforts with crystallographers investigating the atomic structure, we utilize XFEL:s for measuring X-ray absorption and emission spectra to probe the electronic structure of the samples. With this we have a very element-specific tool, capable of singling out manganese atoms from a large number of lighter elements. I will discuss the underlying principles and recent results using L_2,3absorption and Kβ emission spectroscopy, which provides information on the unoccupied and occupied electronic states, respectively. From this we can investigate the oxidation states and spin states of the OEC while going through the light-harvesting cycle.