Hi Sebastiano,
I study a binary alloy (Fe98.7Cu1.3) where a phase change from bcc to fcc is evident (at the Cu K edge of course) and it depends on the annealing time. I have 4 samples A,B,C,D, where the A shows a perfect bcc structure, while the D a clear fcc structure. B and C have structure that is a mix between the fcc and bcc. I found very interesting to perform some linear fit simulations using the A and D of the XANES and EXAFS of the samples B and C. so for instance it is possible to get the xanes of the B sample as linear combinations of A and D according to
BXANES = x*AXANES+(1-x)*DXANES and for the EXAFS
BEXAFS = y*AEXAFS+(1-y)*DEXAFS
The simulations work perfectly but:
1- Of course I expect that x=y (am I wrong?). but this is not true according to the values that the linear fit provide me. There is a small but significant difference as x=48±2% and y=44±2%. Does this difference have any physical meaning? Is the error a bit too small (so should I increase it?)
I don't think you are either wrong or right -- it depends upon the details of the system. Were your system a mixture of two powders in the ratios you observe in sample B and C, then you would indeed expect x=y. If I understand your situation correctly, you have an alloy that displays a local structure (FCC-like around the Cu atom) that is different from the bulk structure (presumably BCC since this is mostly Fe). That means that, on some length scale, the correlations in the material change from FCC-like to BCC-like. Suppose that length scale at which the correlations change is 10s of Angstroms. That is too far for the EXAFS-with-an-E to see, but near the edge the mean free path of the photoelectron might be long enough to start seeing that effect. Thus the structure probed by the XANES portion of the spectrum might not, in that sense, be identical to the structure probed by the EXAFS. In that case, you might not expect x=y. The extent of their difference is an indication that the XANES is probing the system on a longer length scale than the EXAFS. I admit that what I just wrote is a bit half-baked -- it's just my initial thought upon reading your email. But it seems plausible.
2- For the sample C, I got an amazing simulation adding some noise (0.001), but when I try to export the data (operation: "Write a report") I cannot get the data corrected with the noise (I can get only 6 columns: k, data, fit, residual, sample D, sample A). Is there any way to get the data from the data+noise curve?
Hmmm... I need to look into the soruce code, but I would guess that I just forgot to apply the noise when writing out the LCF fit results. I doubt that it will be a difficult fix and I will let the list know when I have checked the fix into the SVN repository. It is unlikely that I'll have a chance to make a new Windows executable before August, however. As a work-around, you could generate a noise spectrum yourself and add it to the data outside of Athena. It won't be the same noise spectrum that was used in the fit, but I suspect it'll be hard to tell the difference when you make a plot. As I say in the document "The noise is randomly generated using an epsilon that is chosen as a fraction of the size of the edge step." B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 My homepage: http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/