Appendix 4 - Reference spectra

Summary

There is perennial interest in reference XAFS spectra. This appendix contains a small sample of µ data taken by the author and collaborators. Preedge fits are removed to improve visibility, but that is all.

The first three spectra are from study of multiple scattering in gases. A Si(111) monochromator was used on a bend magnet beamline (X9) at NSLS. This study permitted isolation of the Cl multiple scattering signal in molecular gases simply by taking linear combinations of raw spectra. Weak backscattering from hydrogen was also noted. The triangle path scattering is evident right above the “white line”; similar features are seen in ZnS₄ tetrahedral sites such as are present in ZnS and various enzymes, such as Aspartate Transcarbamoylase. The GeCl₄ spectrum contains single scattering plus multiple scattering among the Cl; GeH₃Cl contains mostly single scattering from the Cl and weak scattering from H, (the weak multiple scattering between the central atom and the Cl and multiple scattering among the hydrogens are neglected). GeH₄ principally has only weak backscattering from the hydrogens.

The other spectra were taken with a Si(220) monochromator and an 18 KG wiggler at SSRL. The next three spectra are KMnO₄ at 80K, 160K, 300K sample temperature. Note the temperature dependence of the high frequency structure at the edge. This is due to single scattering contributions from non-nearest neighbor atoms, which are loosely bound, and therefore have large temperature dependent σ².