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Theoretical and Experimental Progress on a Quantitative Description of NEXAFS and RIXS in Chemical Compounds

Kolloquium der Abteilung 7

Standard investigations using x-ray absorption (XAS) and emission spectroscopy (XES) to understand electronic structure in chemical compounds, particularly insulators, are normally carried out at a qualitative level. Calculations involving molecular orbitals may allow us to understand gross features observed in XES, but are often far from accurate. We have carried out high-precision investigations of nitrogen compounds using XAS and RIXS around the N K edge on the PTB U49 beamline at BESSY II. We were prepared to match the data with theoretical treatments that took into account disorder due to phonons, dynamical screening of the core and valence holes, exciton production, and non-radiative transitions. An accurate calculation of XAS and RIXS using LDA band structure, screening corrections evaluated with the GW approximation, disorder due to phonon modes of the crystal, and exciton behavior determined from the Bethe-Salpeter equation allows us to understand the observed spectra at a level approaching quantitative accuracy. On the experimental front, new x-ray microcalorimeter detectors are being developed at NIST. They cover a wide energy range and deliver increasingly good resolution for XES. Multi-pixel detectors now give count rates that are promising for synchrotron radiation experiments.