Curved Crystal Transmission Optics for Energy-Dispersive X-ray Absorption Spectroscopy

The development of a curved crystal monochromator of the Laue type for energy-dispersive X-ray absorption spectroscopy is presented. The quality of the X-ray absorption spectra at high photon energies is compared with spectra measured with silicon crystals in the more frequently used Bragg geometry. In the Bragg case, an asymmetric broadening of the reflectivity profile leads to strong distortions of the near-edge fine structure and to a reduction in spectral resolution. The reflectivity profiles of fiat and curved crystals for Laue and Bragg geometry have been calculated using dynamical theory and are compared with experimental data. The new optics have been used for in situ time-resolved X-ray absorption spectroscopy. An example of the application of the technique for the characterization of a Pd catalyst is given. The X-ray absorption fine structure at the Pd K-edge has been measured during the activation and during the heterogeneous catalytic oxidation of carbon monoxide.