Structural Applications of X-ray Absorption Spectroscopy (Exafs and Xanes) in Coordination Chemistry

Abstract

The aim of these lecture notes is to make the reader more familiar with the conditions under which accurate structural information can be extracted from both EXAFS and XANES spectra. Following a recent and elegant theoretical contribution due to Natoli et al. [l4,25] a unifying interpretation scheme of the whole X-ray absorption spectrum is given using a single electron excitation theory with multiple scattering of the photoelectron by a cluster of atoms. XANES spectra are thus shown to be governed by scattering resonances (cage resonances) of the cluster and by a so-called “full multiple scattering”regime. Conversely, EXAFS spectra are dominated by single scattering processes but higher correlations can also be detected and used for practical purposes under conditions which we discuss. The successive steps of standard analyses of the EXAFS spectra are then detailed. It is shown how “extended continuum” MSW-X calculations can be helpful for extracting specific information from edge or XANES spectra. Cases of ill-conditioned EXAFS problems are discussed and it is proven, using several illustrative examples taken from porphyrin chemistry, that difference FT analyses taking specific structural perturbations into account open new paths to unravel the hidden structural information from EXAFS spectra. Selected applications concerning biomimetic complexes of cytochrome P-450 or polymeric chains of organic conducting materials are presented. These lecture notes close with considerations regarding experimental techniques. Exciting time resolved experiments are now becoming possible with energy dispersive spectrometers but emphasis will be laid on the specific advantages of energy scanned spectrometers, i.e. the possibility of recording X-ray excitation spectra using radiative (non radiative) secondary emissions: X-ray fluorescence for low dilution and X-ray excited luminescence for site selectivity.