[Ifeffit] Au-foil (fcc) splitted peak

grant bunker bunker at iit.edu
Sun Oct 9 14:38:39 CDT 2011

Julian - the scattering amplitudes of all elements have structure in them that are connected to electron scattering  (Ramsauer-Townsend) resonances within the backscattering atom.  For light elements (e.g. O, S, Fe) they occur at energies that are lower than the typical transform range, so they usually are ignored.  For heavier scatterers like Au there are strong minima and maxima in the Au scattering amplitude over the transform range. From a signal processing point of view, these amplitude modulations look like beats, which normally are due to different distances interfering.   In the case of Au back scatterers, even if there is only one physical distance, you will get multiple peaks (perhaps overlapping).   Mathematically the fourier transform is the distance distribution convoluted with the fourier transform of the amplitude.   

So, how do you deal with it?  First, if you know what's going on that may be enough.  Alternatively, if all the scatterers are the same, it's not difficult to divide out the scattering amplitude and subtract out the scattering phase before the transform to obtain an "optical transform", which eliminates the effect you're seeing. I don't know if that is supported by ifeffit but it's not hard to do.  This approach is less useful if you have multiple types of scatterers (e.g. Au, S) because if you do the compensation for one type of scatterer it screws it up for the others.  Techniques like regularization are probably better in that case.

hope that helps

grant bunker

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