Hi Iraida, I hope you don't mind that I CC my reply to the ifeffit mailing list. I have some suggestions for you, but I thought it might be helpful to you to see if anyone else has anything to contribute. (The attachments to Iraida's email included a couple of screenshots showing the plotting window with a very good-loooking fit. She also sent the artemis log file showing many of the numbers she quotes below.) On Tuesday 06 April 2004 12:12 pm, Iraida Demchenko wrote:
I have a problem with fitting of my data for 0.5 microns of Ge on Si(001) substrate (grown by MBE). The spectrum for K-edge of Ge was collected by TY mode. This sample is a standard for my next fitting. >From this fitting (for 1 shell) I want to draw So^2... But, if I apply a "trick" with k-weight determination as 1 and So^2 value changing from 0.7 to 1.1 and repetition of this operation for k^1, k^2, k^3, I get three parallel lines for sigma^2(Ge-Ge) as function of So^2 :(. They aren't crossed in this area.
This is kind of surprising to me. I have used the three-lines trick many times and have never seen the situation of three parallel lines. I am a little confused how that is even possible. One might expect this trick to do strange things in the case of a first shell that is not well separated from higher shells, but my memory of Ge is that the first shell is well separated. Another situation that might lead to trouble is if strain induced by the substrate splits the first shell into two well-separated peaks. However, that does not seem to be the case for you either. 1/2 micron is (I think) thick enough so that the film should be bulk-like. Sadly, I am stumped. Has anyone else ever seen this?
On the other hand, I tried to fit my data (3 shells, k-w=2; dk=2; Hanning; k(3.5-12.3)) and I've obtained: reduced chi-square=10.33; R-factor=0.011; So^2=1.22 +/- 0.34; dr_1=0.0107+/-0.006; sigma^2_1shell=0.0037+/-0.0019; sigma^2_2=0.0068+/-0.0033; sigma^2_5=0.0096+/-0.0041(3 shell) (of course, that errors were recalculated(following FEFFIT, p.44). I doubt about the obtained value for So^2=1.22 +/-0.34. Within the error, of course, I can say, that this value is in the range from 0.6 to1. But, the correlations between variables are: amp (So^2) and sigma2_1: 0.916 :(; amp and sigma^2_2: 0.498; amp and sigma^2_5: 0.49; sigma^2_1 and sigma^2_2: 0.47; sigma^2_1 and sigma^2_5: 0.45.....all other correlations are below 0.25. In the attachment there is a log file for this fitting, as you can see sigma^2_3 is -0.0055+/-0.016 :(.
Are there any other ways to determine our So^2? Or these actual results (So^2=1.22) we can consider as good? What do You think about this fit?
First, as you say, your value for S02 is not unreasonable in that the error bar covers a value that one might expect. Still, you are right that 1.22 is an oddly large best fit value and suggests that something else might be happening in the data. Second, I wouldn't be so concerned about the high correlation between S02 and sigma^2. It's normal. Here is a nice short piece by Matt about correlations: http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2004-February/000713.html The rest of thread might be of interest to you, as well. At another point in that thread, there is some discussion of how the normalization is done. Have you examined that carefully? If the post-edge line intersects E0 at an oddly low value, then the edge-step used in the normalization might be rather too small. This is certainly a possibility in fluorescence data where the region after the edge sometimes sweeps upward. In that case, the normalization range is such that the regressed post-edge line can be below the data near E0. I explained this in some more detail at http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2004-February/000711.html That could be the cause of a best-fit S02 that is larger than 1, That's my best guess, given that I have not seen the mu(E) data, for what is happening. I was bit confused by thing about sigma^2_3. I saw in the log file that it was, as you said, negative. But none of the sigma^2 values for the paths were negative. It wasn't clear how that sigma^2 value was being used. Hope that helps, B P.S. I could tell from the log file that you sent me that you are using a *very* old version of Artemis and presumably of ifeffit as well. You may want to download the latest ifeffit installation package from http://cars9.uchicago.edu/ifeffit/download.html and then let the automatic updater bring your computer completely up to date. -- 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/