X-ray absorption spectroscopy (XAS) is ideal to study the structure of molecular systems because it is chemically selective and can be implemented in any type of medium, in particular in liquids, which are the medium of most chemical and biological systems. X-ray absorption near-edge structure (XANES) and X-ray absorption fine structure (EXAFS) in laser pump/X-ray probe experiments allows the retrieval of not only the local geometric structure of the system under study, but also the underlying electronic structure changes that drive the structural dynamics. A time resolution of ~70 ps is reached at present synchrotron X-ray sources, but time-resolved XAS is not limited by the time scale of the X-ray pulse. Indeed, using the femtosecond slicing scheme, which produces ultra short X-ray pulses at synchrotrons, we are able to directly observe dynamical processes including coherent nuclear wave packet motion and ephemeral transitions states of complex chemical and biological systems.
We have pioneered and implemented picosecond and femtosecond XAS [1-4] with which we studied photoinduced electronic (charge transfer, spin changes) and structural changes of metal-based molecular complexes in solutions. The processes we investigated so far include: intramolecular electron transfer in Ruthenium polypyridine and Rhenium-carbonyl complexes, spin cross-over in Fe(II)-based polypyridine complexes[6, 7], bond formation in bimetallic complexes and solvation dynamics around atomic anions in solution.
We recently extended time-resolved optical pump/XAS probe experiments towards data collection at MHz repetition rates. The use of a high-power picosecond laser operating at an integer fraction of the repetition rate of the synchrotron storage ring allows exploitation of up to two orders of magnitude more X-ray photons than in previous schemes based on the use of kHz lasers. Consequently, we reached an order of magnitude increase in the signal-to-noise of time-resolved XAS of molecular systems in solution. This development opens new venues to investigate highly dilute samples where the concentration approaches physiological conditions in case of biological systems. The simplicity and compact nature of the scheme should allow for straightforward implementation at any synchrotron beam line, permitting it to be used with a range of experimental probe techniques, such as time-resolved X-ray diffraction or X-ray emission studies.
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 W. Gawelda, C. Bressler, M. Saes, M. Kaiser, A. Tarnovsky, D. Grolimund, S. L. Johnson, R. Abela, and M. Chergui, Physica Scripta T115 (2005) 102.
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 C. Bressler and M. Chergui, Annual Review of Physical Chemistry, Vol 61 61 (2010) 263.
 W. Gawelda, M. Johnson, F. M. F. de Groot, R. Abela, C. Bressler, and M. Chergui, Journal of the American Chemical Society 128 (2006) 5001.
 W. Gawelda, V. T. Pham, M. Benfatto, Y. Zaushitsyn, M. Kaiser, D. Grolimund, S. L. Johnson, R. Abela, A. Hauser, C. Bressler, and M. Chergui, Physical Review Letters 98 (2007)
 C. Bressler, C. Milne, V. T. Pham, A. ElNahhas, R. M. van der Veen, W. Gawelda, S. Johnson, P. Beaud, D. Grolimund, M. Kaiser, C. N. Borca, G. Ingold, R. Abela, and M. Chergui, Science 323 (2009) 489.
 R. M. van der Veen, C. J. Milne, A. El Nahhas, F. A. Lima, V. T. Pham, J. Best, J. A. Weinstein, C. N. Borca, R. Abela, C. Bressler, and M. Chergui, Angewandte Chemie-International Edition 48 (2009) 2711.
 V. T. Pham, W. Gawelda, Y. Zaushytsin, M. Kaiser, D. Grolimund, S. L. Johnson, R. Abela, C. Bressler, and M. Chergui, Journal of the American Chemical Society (2007)