XANES multiple scattering

Fig. 15. Pictorial view of the scattering processes of the excited internal photoelectron determining the EXAFS oscillations (single-scattering regime) and the resonances in the XANES (multiple-scattering regime). From Bianconi (30).

Fig. 5. Schematic view of the multiple scattering pathways in the EXAFS single scattering regime and in the XANES multiple scattering regime...

The quantification of the extra-framework titanium species in titanium silicalites of MFI structure, TS-1, was performed using either XANES at the Ti K-edge or XPS Ti (2p) photolines. In addition, two different framework sites, [Ti(OH)(OSi)3] and [Ti(OSi)4], were characterized in dehydrated samples using Diffuse Reflectance UV-visible, multiple scattering analysis of EXAFS, H and Si NMR spectroscopies. 

Briefly, XANES is associated with the excitation process of a core electron to bound and quasibound states, where the bound states interacting with the continuum are located below the ionization threshold (vacuum level) and the quasibound states interacting with the continuum are located above or near the threshold. Thus, XANES contains information about the electronic state of the x-ray absorbing atom and the local surrounding structure. However, as stated above, unhke EXAES, since the excitation process essentially involves multielectron and multiple scattering interactions, interpretation of XANES data is substantially more complicated than that of EXAFS data. 

Fig. 1. X-ray absorption spectrum (XAS) of Cu—Zn metallothionein at the Cu and Zn K-edges. The structure near the edge, referred to as XANES is dominated by multiple scattering events while the extended structure, referred to as EXAFS, at photoelectron energies greater than 30-50 eV is primarily due to single scattering events...

The spectral features of XANES have been interpreted as the result of multiple-scattering resonances of the low kinetic energy photoelectrons. Examples of the strong and sharp XANES peaks above the continuum threshold and below the beginning of the weak EXAFS oscillations in the absorption spectra of condensed molecular complexes, are shown in Fig. 4.6. 

The FEFF8 code31 was used to perform self-consistent full multiple scattering calculations. FEFF8 implements self-consistent field potentials for the determination of the Fermi-level and the charge transfer. Full multiple scattering (FMS) calculations are used to calculate the X-ray Absorption Near-Edge Structure (XANES) and the local density of states. 

Calculation of the multiple scattering by means of a real-space cluster approach is considerably more flexible than band-structure methods. Since this technique does not rely on crystal periodicity, it can readily be applied to interpret data for materials of arbitrary atomic arrangements. The sensitivity to higher order correlations has been shown. Fujikawa et al. (94,96) favor short-range-order multiple-scattering XANES theory, in which atoms are not divided into shells but the scattered waves are classified into a direct term and a fully multiple-scattering term. 

Fig. 21. Multiple-scattering calculations for different cluster sizes of fee Cu a, 1 shell b, 2 shells c, 3 shells d, 4 shells e, experimental XANES spectrum. The shells consist of 12 atoms at 2.55 A (first shell), 6 atoms at 3.61 A (second shell), 24 atoms at 4.42 A (third shell), and 12 atoms at 5.09 A (fourth shell). From Greaves er al. (105).

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