SEMINAR      Monday 3 October 2005 - h. 15:00  Auditorium B. Touschek     M. Chergui (Lausanne-EPFL) Ultrafast structural dynamics in chemical and biological systems Abstract    Ultrafast spectroscopy allows to unravel the primary events of a chemical reaction or a biological function.    In this presentation, after recalling the basic principles, I will present recent results on the real-time measurement of light-induced electric field changes within retinal proteins (in this case, bacteriorhodopsin), and relate them to the structural changes, which govern the biological function.    In order to probe the structural dynamics in molecular systems, we also use a laser pump/ X-ray probe scheme in the picosecond time domain, based on X-ray absorption spectroscopy (XAS).    This allows the observation of the changes in the electronic structure (occupancy of valence orbitals, oxidation state, etc.) and geometric structural changes in the system. We will present the results of a picosecond XAS study of Ruthenium tris-bipyridine, where we could determine in detail the bond changes in the excited state.    This study opens the way to a systematic investigation of coordination chemistry compounds and metalloproteins.