X-ray absorption spectroscopy

This review gives a brief description of the theory and application of X-ray absorption spectroscopy, both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), especially, pertaining to photosynthesis. The advantages and limitations of the methods are discussed. Recent advances in extended EXAFS and polarized EXAFS using oriented membranes and single crystals are explained. Developments in theory in understanding the XANES spectra are described. The application of X-ray absorption spectroscopy to the study of the Mn4Ca cluster in Photosystem II is presented

Collecting XAFS spectra at soft X-ray energies in a heated loop cell up to 1600 K

International audienceA new device to collect in-situ-, high-temperature XAFS spectra is presented which runs at low x-ray energies (K edges from Na to Cl). XAFS data is collected in the fluorescence mode (Ge solid state detector) using a heated loop that contains the sample. The actual temperature is measured using an optical pyrometer and a thermocouple (temperatures are +/- 20 K at 1500 K). The first XAFS spectra were collected at the Si K-edge, with 0.2 eV steps, on the ABS1 spectrometer at Super-AGO (LURE, Orsay). The study of the melting of a Na2Si2O5 glass is an example of the possibilities offered by this new area of research

XAS structural and electronic investigation of a new type of catalyst: AlGaPON

Substitution of oxygen by nitrogen in galloaluminophosphate oxides leads to compounds, with properties and applications improved as compared to the oxide. The structural and electronic changes induced by the anionic substitution of oxygen by nitrogen on those amorphous compounds have been studied by XAS. The investigation of the EXAFS and XANES data have allowed us to highlight some structural changes; in particular the existence of mixed tetrahedra XOxNy (x being P, Al or Ga) have been demonstrated. Such a study is helpful in understanding the variations in the surface acido-basic properties of these compounds

Structural investigation of mixed nitrided galloaluminophosphates "AlGaPON"' by EELS, XAS, and XPS spectroscopies

Amorphous nitrided galloaluminophosphates "AlGaPON" catalysts with nitrogen contents varying from 0 to 23.3 wt% N were obtained by nitriding an Al0.5Ga0.5PO4 precursor under ammonia flow at 750degreesC in a tubular furnace. The structural changes induced by this treatment were investigated by electron energy loss spectroscopy (EELS), X-ray absorption spectroscopy (XAS), and X-ray photoelecton spectroscopy (XPS). XANES and XPS results indicate that the first-coordination spheres of P, Ga, and Al atoms are modified by nitridation. In particular, the comparison of the P XANES spectra recorded on "AlGaPON" and on a PON phosphorus oxynitride (reference of mixed PO2,N-2, tetrahedra) reveals that mainly PO2,N-2, tetrahedra are present in highly nitrided samples. Moreover, the replacement of oxygen by nitrogen probably concerned P-O-Ga bonds rather than P-O-Al. EELS investigation reveals that the precursor is homogeneous at the used probe scale, but indicates that nitridation is accompanied by a loss of homogeneity of the material. (C) 2002 Elsevier Science