Time-resolved X-ray absorption fine structure (XAFS) spectroscopy with picosecond temporal resolution is a new method to observe electronic and geometric structures of short-lived reaction intermediates. It combines an intense femtosecond laser source synchronized to the X-ray pulses delivered by a synchrotron Swiss light source (SLS). We present key experiments on charge transfer reactions as well as spin-crossover processes in coordination chemistry compounds next to solvation dynamics studies of photogenerated atomic radicals. These examples emphasize the observables at hand using ultrafast XAFS techniques, which include the density of states, full and even partial changes in oxidation state, and internuclear distances with milli-Angström accuracy. An outlook towards femtosecond studies and biologically relevant systems stresses the high potential of XAFS methods using new femtosecond X-ray sources like free electron lasers (XFELs).
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