On Thursday 06 May 2004 06:05 pm, chachi rojas wrote:
I am a graduate student in physics at San Marcos University, Lima,Peru and I'm interested in studying the intermetalic alloys Fe2Zr and Fe3Zr by EXAFS (K-Fe-edge), this samples was prepared by mechanical milling. I am a beginner in this technique (EXAFS), I try search some information about these system, analysed by EXAFS. I use IFEFFIT (feff6) and Artemis programs on Linux Slackware. Bien, in my fit i find e0= -45.34 +/- 2.15, that is a error??.. a fit the first shell in Fe2Zr system. please help me, if is posible, send me some information, about these systems analysed by EXAFS or some references on it, specially Fe2Zr. I really appreciated it if you could help me. Thanks Chachi
Chachi, This is a very good question. In fact, I think it is important enough that I am taking the liberty of CCing my response to the ifeffit mailing list (pues, escribiendo en ingles y no en espanol). This is most definately a problem. A huge E0 like that is probably coincident with an unphysical value for one or more bond lengths. I could explain, but fortunately Alain Michalowicz gave an excellent talk on this very topic a while back at NSLS. Here is the PDF of his talk, which provides an excellent overview of the "large-E0" issue: http://cars9.uchicago.edu/xafs/Advanced_Methods/BNLworkshop2001/michalowicz.... So that helps explain what went wrong, but not how to make it right. Here are some suggestions and I imagine that some folks on the mailing list might have more: 1. Make sure that your E0 was set sensibly and calibrated to a sensible value way back in Athena. Metallic zirconium tends to have the annoying feature of a double peak in the first derivative spectrum and typically the second peak is larger than the first. (That is true both in pure Zr and in Zr-alloyed Sm2Co17.) Consequently, the algorithm used by Ifeffit (and so by Athena) to automatically determine E0 will choose the second peak, since it is bigger. This places E0 10s of volts above the real E0, thus introducing an enormous E0 shift into the data. Your fit might be conpensating for that or your fit might simply be resolving to a non-physical minimum of chi-square. (I know you said you are measuring the Fe edge, but a similar thing could happen elsewhere.) 2. There is documentation under Athena's Help menu about how to use the calibration tool in Athena. There is also a demo project (also under the Help menu) explaining how the calibration tool works. Make sure you know how this is done. 3. If your data are very noisy for any reason, the algorithm used by ifeffit to find E0 could be confused by the noise. It is even possible that it will choose an E0 value well in the pre-edge if the pre-edge is noisy enough. So watch out for that. 4. Last month, there was an interesting discussion on the mailing list about background removal and noisy spectra. It covered a lot of ground, but Shelly's response: http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2004-April/000860.html includes an excellent point about the relationship of E0 in Athena and the later fit in Artemis. 5. Finally, it is possible that you need to take a more cautious approach to using Artemis. Have you read this web page: http://leonardo.phys.washington.edu/~ravel/software/exafs/startfitting.html It outlines my philosophy for how to start doing a fit using feff, ifeffit, and Artemis. By adopting this stategy and slowly building upi the parts of your fiting model, it is much easier to avoid some of the most common pitfalls. Hope that helps, B -- Bruce Ravel ----------------------------------- ravel@phys.washington.edu Code 6134, Building 3, Room 405 Naval Research Laboratory phone: (1) 202 767 2268 Washington DC 20375, USA fax: (1) 202 767 4642 NRL Synchrotron Radiation Consortium (NRL-SRC) Beamlines X11a, X11b, X23b National Synchrotron Light Source Brookhaven National Laboratory, Upton, NY 11973 My homepage: http://feff.phys.washington.edu/~ravel EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/