Hello, I am trying to fit an EXAFS spectrum of an X-ray amorphous Fe precipitate. In a model (goethite), there are Fe atoms at 3.02, 3.29, and 3.45 A. Fitting the two most distant shells of iron gives strange results. Fitting them simultaneously gives very high amplitudes (32, -66) which cancel each other (fig. 1) and produce a beautiful and unreasonable fit (fig. 2). Fitting them separate gives small ('reasonable') amplitudes, but the distance (dr) have very high uncertainties, e.g., dr = 0.12 +- 20.3. Can I expect to resolve these three Fe shells or is it asking too much? I am also attaching the Artemis file, but will appreciate any comments and suggestions. Juro Majzlan ---------------------------------- Juraj Majzlan Institute of Mineralogy and Petrology Albert-Ludwig-University of Freiburg Albertstrasse 23b Freiburg, D-79104, Germany telephone +49-761-203-6416 fax +49-761-203-6407 http://www.minpet.uni-freiburg.de/sites/personen/majzlan.html
Hi Juro, Off-hand, it sounds like you might be trying to fit a square peg into a round hole, so to speak. If the precipitate is amorphous, do you have any reason to strongly suspect a goethite-based structure? I must admit, I haven't looked at your Artemis file yet, so I'm not positive what you mean by "fitting them separate" as opposed to "simultaneously," but you're probably not going to be able to resolve peaks that close...one commonly used criteria is that delta k > pi/[2(r2-r1)] where r2 and r1 are the locations of the peaks and delta k is the width of the k-range you're sampling. So the 0.16 A difference between the higher two points requires delta k > 10 A-1 which your data probably doesn't quite support. The fact that you get high uncertainties in the distances when you fit them separately tends to confirm that. By the way, note that the inability to resolve peaks doesn't mean you should lump them together when you know (or strongly suspect) they should be there...it just means you can't have separate parameters for them. For instance, two nearby peaks that are treated as one will have an anomalously large sigma^2 and possibly an anomalous third cumulant as well...if you have a good enough model, it is better to treat them as separate peaks with the same number of free parameters (e.g. constrain their sigma^2's to be the same, and perhaps fix the separation between them or something like that). But in your case I'm not sure you have strong reason to expect there really are separate peaks... --Scott Calvin Sarah Lawrence College At 03:46 PM 9/26/2005 +0200, you wrote:
Can I expect to resolve these three Fe shells or is it asking too much?
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