Bruce, Peter,
Athena and Artemis do not plot chi beyond 20 �-1 or the Fourier transform beyond 10 �. Is this a limitation of Athena/Artemis or of Ifeffit? Can it be changed?
This only seems to be half true. The fe.060 file that comes with the Athena examples has data out to 22 invAng and gets plotted correctly by both programs.
The 'chi(k) only goes to k=20' issue might be due to the fact that the Feff calculations don't normally go past k=20. That makes it hard to do a fit that far out! I think this could be changed, but it would take a little time. Are you using Feff6l or Feff8?
On the other hand, you are correct that data only gets plotted in R up to 10 A. That was not a conscious decision by me and seems to be some kind of default in Ifeffit. Matt, how does one change that default?
You can set the maximum R for chi(R) from fftf() with fftf(data.chi, kmin=2,kmax=19, rmax_out=15)
The 10.5 � peak raises another issue. I understand that Ifeffit by default assumes that the high-R region is pure noise in order to make its estimates of uncertainty. (I know the actual R-range that is used was mentioned recently but I can't find it in the list archives or any of my manuals.) Can this range be changed? Does anyone have any advice on how to deal with this issue?
Just to add to Bruce's notes: The prevailing wisdom is that the automated estimate of epsilon_k is generally too low -- that makes chi_square too large. Having 'real data' out there might actually help!! But simply setting epsilon_k to an appropriate value is probably the best bet. Remember that epsilon_k is the uncertainty in unweighted chi(k), and so is fairly easy to interpret (ie, no units). Artemis reports this as 'Measurement uncertainty (k)' in the Results window after a fit but you can overwrite it with the Epsilon value on the page with the data file and Fit ranges. A reasonable number for most good EXAFS would be epsilon_k = 0.001. --Matt