By making a comparison between the Mn K-edge absorption of (${\mathrm{MnO}}_4$ ${)}^{\mathrm{-}}$ and [Mn(${\mathrm{H}}_2$O${)}_6$ ${]}^{2+}$ complexes in aqueous solution we obtain an experimental determination of the energy extent of the type-II multiple-scattering (MS) regime that is substantially wider than expected. Theoretical calculations based on the MS formalism support this conclusion. We also recognize three energy regions in the absorption spectra of these complexes: a full MS region, where numerous or an infinite number of MS paths of high order contribute (depending on whether the MS series converges or not), an intermediate MS region, where only a few MS paths of low order are relevant, and a single-scattering region where the photoelectron is backscattered only once by the ligands [extended x-ray-absorption fine-structure (EXAFS) regime]. Theoretical considerations show that this must be a general situation in x-ray-absorption spectra and opens the way to a unified scheme for their interpretation. The energy extent of the three regions is obviously system dependent. We also show how to generalize to MS contributions the usual EXAFS analysis using curved-wave propagators and indicate how to extract geometrical information from the spectra of the two clusters investigated. In particular the method is used to derive the Mn—O—O—Mn path length in the (${\mathrm{MnO}}_4$ ${)}^{\mathrm{-}}$ complex.

The oxygen K-edge x-ray-absorption near-edge structure of stoichiometric NiO has been measured, using partial electron yield, at the ‘‘grasshopper’’ line of the Frascati synchrotron-radiation facility. The main features of the spectrum in the multiple-scattering region over the 50-eV energy range beyond threshold are well predicted by the full multiple-scattering one-electron theory. The interpretation of the fine details of the spectrum requires a many-body picture, in fact, the results are in agreement with a description of NiO as a system that cannot be classified as a standard mixed-valence system.

The geochemical behavior of sulfur in magmas depends strongly on the oxidation state of sulfur, but this is not easily deter- mined by standard analytical methods. We have measured XANES absorption spectra at the sulfur K-edge and have found that such measurements are useful to characterize the oxidation state and speciation of sulfur in silicate glasses of geological rel - evance. Measured spectra of a set of reference minerals show the effects of different oxidation states and coordination numbers of sulfur; there is a large shift in energy (~10-12 eV) of the sulfur K-edge between S2- and S6+. This large and easily detectable difference makes possible the measurement of the valence of sulfur in unknown samples by measuring the shift in energy of the absorption edge. This approach is applicable to both crystalline and glassy materials, and useful results have been obtained on samples with as little as 450 ppm S. We have used XANES measurements to characterize oxidation state and speciation of sulfur in a set of natural and synthetic sulfur-bearing glasses. The samples cover a range of composition from basaltic to almost rhyo litic, and some were synthesized over a range of pressure, temperature and oxygen fugacity; glass S content varies between 450 and 3000 ppm. XANES analyses, carried out in fluorescence mode at LURE, allowed determination of the sulfur oxidation state in all of the samples and clearly show that some samples contain a mixture of S 2- and S6+; no other sulfur species were observed. Quantitative determination of the abundance of sulfide and sulfate shows good agreement with independent measurements based on electron-microprobe determination of the wavelength shift of sulfur K X-rays.

We have investigated the electronic states of highly oriented pyrolitic graphite and single-walled carbon nanotubes using x-ray absorption spectroscopy (XAS) before and after annealing treatment in ultrahigh vacuum, and observed that the small peak between ${\pi }^{*}$ and ${\sigma }^{*}$ features, which has been previously assigned to free-electron-like interlayer states, disappears after in situ annealing treatment, suggesting that the signal may be assigned to a surface contamination, especially oxygen contamination introduced by chemical processing or gas adsorption. Additional experiments by photoelectron spectroscopy as well as XAS methods, performed after aging in air, fully support this interpretation.