XAS and X-ray Magnetic Circular Dichroism (XMCD) are probes of local order and microscopic magnetic properties. XMCD is a selective probe, which can access to a large variety of elements. The dispersive EXAFS station at SOLEIL : ODE beamline gives the possibility to perform numerous pressure XAS and XMCD experiments with an excellent statistic and very fast XAS kinetic.
The classical method of recording absorption spectra is the step by step measurement of the absorption coefficient for each energy point. A different way was first proposed by Matsuchita, the use of a bent crystal as monochromator to eliminate the stepwise scanning of the X-ray energy. The continuous change of the Bragg incidence along the bent crystal opens an energy range in the reflected beam. The correlation between position and energy of the X-ray is exploited thanks to a position sensitive detector.
The main advantages of Dispersive XAFS are the focusing optics, the short acquisition time (few ms) and the great stability during the measurements due to the absence of any mechanical movement. These three advantages allow the study of small samples, 70mm at SOLEIL, to follow kinetics, and to perform experiments demanding a small signal to noise ratio (typically 105). Small samples are mandatory in the case of high pressure studies, the smaller the sample, the higher the available pressure will be: up to 100 GPa at SOLEIL .
High pressure, possibly combined with high or low temperature, is experimentally produced using diamond anvil cells. In these cells, the sample chamber size is typically 100 mm in diameter and 15 mm in thickness. Thanks to its focusing optic, the X-ray dispersive set-up is well adapted to this type of experiments and allows high pressure XAS since more than 20 years. The real time visualization of the XAS spectra allows to eliminate the parasitic anvil cell diamond Bragg reflections (glitches) by an appropriate alignment.
One of the purpose/advantage of ODE setup is for fast kinetic measurement, because its time resolution is only limited by the readout speed and the photon flux of the incidence X-rays. Microsecond DEXAFS has recently become accessible in ODE beamline, with the advent of the fast readout silicon quantum detector and the high brilliant X-rays produced from synchrotron soleil running at a current of 430 mA. The feasibility was investigated with a prototypical thermally driven redox reaction, the thermal decomposition of a platinum compound. Detailed electronic and geometric structure dynamics of the reactant, intermediate and product was followed with continuous snapshots every 60 μs. This method could also be applied to other reaction dynamics, such as the formation mechanism of metal nanoparticles and catalysis.