High-resolution X-ray spectroscopy of rare events: a different look at local structure and chemistry

The combination of large-acceptance high-resolution X-ray optics with brigh t synchrotron sources permits quantitative analysis of rare events such as X-ray fluorescence from very dilute systems, weak fluorescence transitions or X-ray Raman scattering. Transition-metal K beta fluorescence contains in formation about spin and oxidation state; examples of the characterization of the Mn oxidation states in the oxygen-evolving complex of photosystem II and Mn-consuming spores from the marine bacillus SG-1 are presented. Weake r features of the K beta spectrum resulting from valence-level and 'interat omic' ligand to metal transitions contain detailed information on the ligan d-atom type, distance and orientation. Applications of this spectral region to characterize the local structure of model compounds are presented. X-ra y Raman scattering (XRS) is an extremely rare event, but also represents a unique technique to obtain bulk-sensitive low-energy (<600 eV) X-ray absorp tion fine structure (XAFS) spectra using hard (<similar to> 10 keV) X-rays. A photon is inelastically scattered, losing part of its energy to promote an electron into an unoccupied level. In many cases, the cross section is p roportional to that of the corresponding absorption process yielding the sa me X-ray absorption near-edge structure (XANES) and extended X-ray absorpti on fine structure (EXAFS) features. XRS finds application for systems that defy XAFS analysis at low energies, e.g. liquids or highly concentrated com plex systems, reactive compounds and samples under extreme conditions (pres sure, temperature). Recent results are discussed.