• Time-resolved x-ray absorption spectroscopy: Watching atoms dance
    C.J Milne, V.-T. Pham, W. Gawelda, R.M Van der Veen, A. El Nahhas, S.L Johnson, P. Beaud, G. Ingold, F. Lima, D.A Vithanage, M. Benfatto, D. Grolimund, C. Borca, M. Kaiser, A. Hauser, R. Abela, C. Bressler and M. Chergui
    Journal of Physics: Conference Series, 190 (2009), p12052
    DOI:10.1088/1742-6596/190/1/012052 | Abstract | Article PDF
The introduction of pump-probe techniques to the field of x-ray absorption spectroscopy (XAS) has allowed the monitoring of both structural and electronic dynamics of disordered systems in the condensed phase with unprecedented accuracy, both in time and in space. We present results on the electronically excited high-spin state structure of an Fe(II) molecular species, [FeII(bpy)3]2+, in aqueous solution, resolving the Fe-N bond distance elongation as 0.2 Å. In addition an analysis technique using the reduced χ2 goodness of fit between FEFF EXAFS simulations and the experimental transient absorption signal in energy space has been successfully tested as a function of excited state population and chemical shift, demonstrating its applicability in situations where the fractional excited state population cannot be determined through other measurements. Finally by using a novel ultrafast hard x-ray 'slicing' source the question of how the molecule relaxes after optical excitation has been successfully resolved using femtosecond XANES.
  • Capturing Transient Electronic and Molecular Structures in Liquids by Picosecond X-Ray Absorption Spectroscopy
    W. Gawelda, V.T. Pham, A. El Nahhas, M. Kaiser, Y. Zaushitsyn, S.L. Johnson, D. Grolimund, R. Abela, A. Hauser, C. Bressler and M. Chergui
    American Institute of Physics Conference Proceedings, 882 (2007), p31-36
    DOI:10.1063/1.2644425 | unige:4058 | Abstract | Article PDF
We describe an advanced setup for time-resolved x-ray absorption fine structure (XAFS) Spectroscopy with picosecond temporal resolution. It combines an intense femtosecond laser source synchronized to the x-ray pulses delivered into the microXAS beamline of the Swiss Light Source (SLS). The setup is applied to measure the short-lived high-spin geometric structure of photoexcited aqueous Fe(bpy)3 at room temperature.
  • Structural Determination of a Short-lived Iron(II) Complex by Picosecond X-ray Absorption Spectroscopy
    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), p57401
    DOI:10.1103/PhysRevLett.98.057401 | unige:3198 | Abstract | Article PDF | Article PS (gzipped)
Structural changes of the iron(II)-tris-bipyridine ([FeII(bpy)3]2+) complex induced by ultrashort pulse excitation and population of its short-lived (≤0.6  ns) quintet high spin state have been detected by picosecond x-ray absorption spectroscopy. The structural relaxation from the high spin to the low spin state was followed over the entire lifetime of the excited state. A combined analysis of the x-ray-absorption near-edge structure and extended x-ray-absorption fine structure spectroscopy features delivers an Fe-N bond elongation of 0.2 Å in the quintet state compared to the singlet ground state.



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