Dear Silvio, Wayne, Yes, what Silvio is seeing is a 'perfectly normal' minimum in backscattering amplitude for scattering from heavy atoms. It actually shows up for many elements with Z>35, but at such low k-value that it is lost in the XANES features, and is really only noticeable in EXAFS for Z>40 or so. It is definitely visible in Ag (Z=47), and for Z>60, it is very strong, with the minimum in scattering at ~5 to 6 Ang^-1. This often gives rise to a 'double peak' in |chi(R)| even thought there is really only one shell. This effect is a function of Z for the *backscatter* -- not the absorbing atom. So it will show up for metallic Hg, but not in the first shell for the Hg edge of HgS. As Wayne said, this is normally called a Ramsauer-Townsend resonance, though that term was originally about electron scattering from noble gases. See http://en.wikipedia.org/wiki/Ramsauer-Townsend_effect for a little more information. The way I think about this is that when the outgoing photoelectron has a wavelength near 1 Ang (ie, k ~=6 Ang^-1), it may be able to "tunnel through" the very deep, very small scattering potential from a very heavy element. This will only happen at a fairly distinct wavelength, and so gives a 'resonant dip in scattering'. Hope that helps, --Matt