On Sunday 16 December 2007 04:08:00 van der Veen Renske wrote:

Hello EXAFS experts,

I started using Athena and Artemis two weeks ago by applying it to the EXAFS analysis of a moderately complicated molecule in solution. The final fit is very good (R=0.006) but the errors on the fitting parameters are very large and there are many high correlations between parameters that shouldn't be correlated (even though I do multiple k-weighted fitting). I think the problem is that there are at least 4 scattering pathways that overlap in the FT (spread in path lengths of +-0.4 Angstrom). In addition, these scattering paths involve different atoms and bonds, so that I have to define different delta_r and sigma_sq parameters for all of them.

My question is now: how could I reduce the error bars of the fitting parameters that belong to these overlapping scattering paths? Or is it hopeless without further information of the system?

Hi Renske, You question is at the very heart of the problem of EXAFS analysis. I hope you won't be disappointed that I do not have an easy answer for you. With Feff and Ifeffit, we treat EXAFS as a signal processing problem in the sense that we consider a band width defined buy the Foruier transform range in k and the fitting range in R. That band width sets some kind of upper limit on the amount of information we can ever hope to extract from the signal. Typically, that is not a very large number. Also typically, we have complicated structural problems about which we are trying to gain insight with that limited information content. The whole trick in using the Fourier transform to help us interpret our data is to rely on different paths contributing different Fourier components to the measured signal. In the case of, say, the first and second coordination shells of copper metal, that works extremely well. In most cases -- not so much because, as you have observed, paths contributing significant spectral weight may have overlapping Fourier components. If all we have is the EXAFS signal itself, we would be stuck where you are right now -- reasonable fits but huge correlations. The only way out of this problem is to rely on other information. Sometimes that other information might be intrinisc to the problem, other times extrinsic. Let me give a couple of examples. These examples may be pretty far away from what you are working on. Since you gave us a very scant description of your problem, I cannot speak directly to it. An example of intrinsic knowledge would be the handling of the short oxygen bond in a uranyl or plutanyl compound. In those cases, there is a dioxo-actinide chain consisting of two double bonded oxygen atoms on either side of the actinide atom forming a linear chain. These bonds are very stiff and contribute a strong multiple scattering signal that often overlaps with single scattering signals around 3 angstroms. Those MS paths do not, however, require the use of new parameters. The bond length and sigma^2 parameters for the MS paths can be easily written in terms of the parameters for the SS paths to those oxygen atoms. In that way, the contribution of those MS paths is added correctly to the fit without having to introduce any new parameters. That is a good thing. Examples of extrinsic knowledge are easy to come across. One example might be the temperature dependence of a sigma^2 parameter in a temperature series measurement, which should behave like a single frequency (or Einstein) oscillator. Another example might be knowledge that you bring to the problem from, say, vibrational spectroscopy emasurements on the same compound. In that case, you would expect EXAFS and the other measurement to be consistent, within the caveat that the time and length scales of the XAS measurement are very short. So, I don't think I have answered your specific question, but hopefully, I have written something useful to you. If you would like to ask a more specific question about your problem, feel free to ask on the list. As you have no doubt seen even in the last two weeks since you started using our codes, folks on the list are very welcoming to newcomers and very generous with useful information. Regards, 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/