Diffraction anomalous fine structure:
Unifying x-ray diffraction and x-ray absorption with DAFS
Larry B. Sorensen, Julie O. Cross, M. Newville, B. Ravel, J. J. Rehr, and H. Stragier
Department of Physics, University of Washington, Seattle, Wa. 98195, U.S.A.
C. E. Bouldin and J. C. Woicik
National Institute of Standards and Technology, Gaithersburg, Md. 20899, U.S.A.
Resonant Anomalous X-Ray Scattering: Theory and
Ed. G. Materlik, C. J. Sparks, and K. Fischer, North-Holland, pp. 389--420, (1994)
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This chapter describes a developing x-ray spectroscopic, structural, and crystallographic method called the diffraction anomalous fine structure technique (DAFS), which measures the elastic Bragg reflection intensities versus photon energy. This new method combines the long-range order and crystallographic sensitivities of x-ray diffraction with the spectroscopic and short-range order sensitivities of x-ray absorption techniques.
In the extended fine structure region, DAFS provides the same short-range structural information as EXAFS: the bond lengths, coordination numbers, neighbor types, and bond disorders for the atoms surrounding the resonantly scattering atoms. In the near-edge region, DAFS provides the same structural and spectroscopic sensitivities as XANES: the valence, empty orbital and bonding information for the resonant atoms.
Because DAFS combines the capabilities of diffraction, EXAFS and XANES into a single technique, it has two enhanced sensitivities compared to the separate techniques: (1) Wavevector selectivity. DAFS can provide EXAFS- and XANES-like information for the specific subset of atoms selected by the diffraction condition. (2) Site selectivity. DAFS can provide site-specific absorption-like spectroscopic and structural information for the inequivalent sites of a single atomic species within the unit cell.
We present the theory, experimental methods, and analysis techniques that we have developed, and we show that they work very precisely for Cu metal. We also show that DAFS can yield its enhanced sensitivities while maintaining a precision comparable to that of the best EXAFS and XANES measurements. Wavevector selectivity is demonstrated with a study of a buried 400Å thick In0.2Ga0.8As layer which is wavevector separated from its GaAs substrate and cap. Site selectivity is demonstrated with a study of the two inequivalent Cu sites in a 2400Å thick YBa2Cu3O6.6 superconductor film.