On Thursday 02 December 2004 02:17 pm, Barton, David (DG) wrote:
Carlo and Matt:
With these files, the location of the edge (and element identification as well) using the first derivative does not work.
I also had a similar problem when we had a somewhat noisy pre-edge and edge data. Often our analysis of the XANES requires high confidence in the edge energy to determine energy shifts and to accurately align our spectra using metal foil references. Of course, I can always manually assign the edge position in Athena, but it would be nice if Ifeffit was a bit smarter about how it chooses Eo since it is critical for Athena operation when a reference standard is used to calibrate a sample spectra. I would guess that there is some robust algorithm that one could implement which would more accurately determine either (1) the maximum in the first derivative or (2) the zero of the second derivative for data with any number of data points or noisy data. I imagine that such an algorithm would include some sort of adjustable "smoothing" parameter to filter the signal from noise.
Carlo's problme seems to be a little bit different. His data are not particularly noisy. The problem appears to be correlated to the length in steps of the pre-edge. I, too, would be interested in a perfectly robust against noise algorithm for finding the edge energy. However, I suspect that when someone builds a better algorithm, somoeone else will build noisier data ;-) Athena does have a feature that might be some help in this case. In the "bkg" section of the preferences, one of the parameters is "e0". This is set to "derivative" by default, telling Athena to use Ifeffit's first derivative based algorithm to determine e0. Unless the noise in the pre-edge is really large, the "half" option for that parameter might be helpful. It's described right there on the preferences page. Another things that might help is Athena's pre-processing of data as it's imported. If all goes well, then you will have to set e0 by hand for the first data set, but choosing the "set all parameter values" option in the processing dialog should set all the rest to reasonable values. In any case, if you have noisy data, you really must be prepared to do hands-on data analysis. Ifeffit and Athena will only ever be so smart, but data can be arbitrarily crappy! 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/