Backscattering atoms, contributions

In the analysis of EXAFS data on bimetallic clusters, we consider two EXAFS functions, one for each component of the clusters (8,12-15.17). If the treatment is limited to contributions of near-est neighbor backscattering atoms, each of the functions will consist of two terms. For a bimetallic cluster composed of elements a and b, the EXAFS associated with element a is given by the expression ... 

In these expressions, the subscript outside the braces identifies the absorber atom, while the superscript identifies the backscattering atom. The contribution xjfK) of one type of backscattering atom to the total EXAFS function is given by the equation ... 

Figure 4. Contributions of nearest neighbor copper and osmium backscattering atoms (circles in fields B and C, respectively) to the EXAFS (solid line) associated with the osmium Ltjj absorption edge of a silica supported osmium-copper catalyst, me circles in field A represent the combined contributions resulting from the data analysis. Reproduced with permission from Ref. 12. Copyright 1981, American Institute of Physics.

For the osmium EXAFS, the first term in Eq. 4.10 represents the contribution of osmium backscattering atoms. In this term, the quantity /V, represents the number of nearest neighbor osmium atoms about an osmium absorber atom and R, represents the distance between the osmium atoms. The phase shift function 28, (/0 is that for an OsOs atomic pair. The quantity f,(/0 exp(—2/C 2tr,2) differs from the analogous quantity for pure metallic osmium by a factor exp(—2X 2Ao-,2), where Ao-,2 is the difference between the value of o-,2 for the OsOs pair in the osmium-copper catalyst and the value for the same pair in the pure metallic osmium. Note that the quantity F,(K) exp( —2/C2cr,2) for the pure metallic osmium is known from the analysis of EXAFS data on it, as indicated earlier. 

As an example, we show in Figure 3 a backscattering spectrum from GaAs (110), obtained vwth a 300-keV Li ion beam. This is a well-chosen test example of energy resolution, as the atomic numbers of the two constituents are quite close (31 and 33 for Ga and As, respectively). Not only are these two species well resolved, but the two common isotopes of Ga are also well separated. Note that the peaks are asymmetric due to contributions from lower layers. Resolving power of this kind surely will find many new applications in materials science. 

Figure 6.13 Rh K-edge EXAFS spectra, uncorrected Fourier transforms according to (6-10) and isolated EXAFS contribution from the first neighbor shell of Rh metal (top), Rh20 ( (middle) and RhCI3 (bottom). The first shell contributions clearly reflect the different backscattering properties of Rh, O and Cl atoms. Note the high number of coordination shells that are visible in Rh bulk metal (from van Zon et al. [35]).

Figure 1.1. Schematic diagram showing the electron elastic scattering pathways contributing to the techniques of low energy electron diffraction (LEED), backscattering photoelectron diffraction (including the scanned-energy mode - PhD) and surface extended X-ray absorption fine structure (SEXAFS). Black disks represent substrate atoms, grey-shaded disks represent adsorbate atoms.

---