December 2011 - Ifeffit - millenia.cars.aps.anl.gov

Au-foil (fcc) splitted peak
by grant bunker 29 Jan '12

29 Jan '12

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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Trouble running Demeter 0.9.6 under Parallels
by Scott Calvin 26 Dec '11

26 Dec '11

Hi all, I'm having trouble running Demeter 0.9.6 under Parallels, and I'm wondering if anyone has had a similar problem, and/or a suggested solution. I have a MacBook Pro running Snow Leopard (10.6.8) and Windows XP SP2 under Parallels 6. Demeter 0.4 was running fine. I attempted to install Demeter 0.9.6, and it appeared to install normally, but when I launched Athena I noticed I was still in 0.4. Some poking around individual files seemed to confirm they had not been replaced. I then tried uninstalling Demeter--it said it was uninstalling 0.9.6, but got rid of 0.4, as I intended. I then installed 0.9.6, which again seemed to work. Upon trying to launch any of the components of Demeter (Athena, Artemis, Hephaestus, Atoms) the splash box appears, sticks around a few seconds, and then disappears, along with the black DOS window. No notice of a crash appears; the program just fails to launch. Any ideas? --Scott Calvin Sarah Lawrence College

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Looking for reviewers for first chapter of new XAFS book
by Scott Calvin 17 Dec '11

17 Dec '11

Hi all, I have finally written the opening chapter of the XAFS textbook I'm working on. This is an introduction and overview, about 30 pages in length. It is (hopefully!) suitable for beginners, although more expert opinions are also welcome. If you are interested in reviewing the chapter and can get feedback to me by 9 January, please contact me directly at SCalvin(a)slc.edu<mailto:SCalvin@slc.edu> . As always, repeat reviewers are welcome! --Scott Calvin Sarah Lawrence College

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