Photoelectron multiple scattering

Multiple scattering, where the photoelectron wave samples severd scatteiers before returning to the absorbCT, is only important in KAFS for cases where two scatterers and the absc rber are nearly coUinear. In such cases, the EXAFS amplitude of the outer scatterer will be significantly enhanced. See Teo, B.K., /. Am. Chem. Soc., 1981,103,3990-4001. 

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...

Several reasons have been put forward to explain the change in the angular intensity pattern of the photoelectrons. One explanation is that intermediate neutral energy levels are ac-Stark shifted into resonance and contribute new selection rules to the photoionization process [53,54], Another possibility is that the electrons of the Kr or D2 are driven into the core Kr+ or D2 in a scattering-like process that creates interference fringes in the photoelectron angular distribution due to interference between multiple scattering channels [55],... 

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. 

N,Sq is a term that modifies the amplitude of the EXAFS signal. The subscript i denotes that this value can be different for each path of the photoelectron. For a single scattering path, N, represents the number of coordinating atoms within a particular shell (at the same radial distance from the absorber). For multiple scattering, N, represents the number of identical paths. The passive electron reduction factor, Sg, usually has a value between 0.7 and 1.0. It accounts for the slight relaxation of the remaining electrons in the presence of the core hole vacated by the photoelectron. 

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. 

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