XANES multiple scattering resonances

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. 

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).

XANES studies of ruthenium-polystyrene hydrogenation catalysts (32) show strong multiple-scattering resonance due to carbon atoms in the first coordination shell which exhibit high backscattering at low kinetic energy of the photoelectron, in accordance with EXAFS results. 

Here we discuss the effect of bond length on XANES spectra. It is well known that EXAFS oscillations are determined by the interatomic distances and coordination numbers. Also a full multiple scattering resonance in the XANES range is strongly affected by interatomic distance. 

The great similarity between the K-edge XANES spectra of transition metals with the same f.c.c. crystal structure shows that the experimental features of the spectra depend mainly on the atomic arrangements of the atoms, and the multiple scattering resonances are only shifted in energy depending on the interatomic distance. 

In the XANES region of the X-ray absorption spectrum, two physical processes play a significant role. These are electronic transitions into unoccupied electronic states and multiple scattering (MS) resonances. 

XANES spectra provide local structure information and the oxidation state. In the XANES region, electrons that are excited from the core level (usually the K level) jump into unfilled bound states, nearly bound states (resonances) or continuum states. Due to multiple electron scattering, XANES spectra are harder to interpret than EXAFS spectra. 

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