I'm still confused about the right way to handle these in practice. It seems that a choice of "arc-tangent", "lorenztian", or "mimic the shape of the main edge" should work for most cases -- do you agree? It also seems there ought to be a reasonable way for the algorithm to pick which of these lineshapes to use. Maybe you`ll find the two graphs in the attachment helpful - they are from our

Dear Matt, paper cited at the top and also accessible on Iztok Arcon`s homepage. The first graph shows the usual range of EXAFS (~ 1000 eV above the edge) on the horizontal scale, and the atomic number on the vertical scale, with inserted energies of the main MPE. You can see that for 3d elements (Ti-Zn) the EXAFS range is almost free of MPE, the prominent 1s3p still within 100 eV of the edge, and the weak 1s2p at the end of the usual EXAFS range. For 4p elements(Ga-Kr) the 1s3p slowly moves into the EXAFS region. And beyond Kr, the strong 1s3d excitation gets beyond the 100 eV mark, so that two prominent MPE are to be taken into account. The L EXAFS are, as seen from the graph, practically all infested by MPE, so that there are no "clean" or "safe" ranges of Z. In the second graph, the "real" atomic backgrounds, extracted from EXAFS spectra, are shown for 4p elements. ALthough very noisy, they show what kind of an ansatz would be necessary to approximate at least crudely their shape. I imagine that two functional terms with proper energy position and amplitude (these are mostly transferable, prepared in tables) but with a small common fittable energy shift (to account for different energy calibrations or small chemical shifts). To allow the usual sloppines in the practical EXAFS analysis regarding the use of proper "matrix" subtraction etc, it would be sensible to retain some smooth background function, either a spline or a polynomial, but with much less free parameters. My feeling is that a 4-interval spline or 4th degree polynomial should be sufficient since the spline would not need to simulate the sharp jumps of MPE. Best wishes Lojz