Eckhard, I am sorry that you did not receive a response to your questions from two weeks ago. The day your message appeared on the mailing list, I was busy at the beamline. Ideally, the point of the mailing list is to draw upon a wider pool of expertise so that if one person is unavailable, another person steps up. It's a good idea.... As you can see I am responding to your original query rather than to the follow up message you sent directly to me. I am not certain that I can answer your questions fully, but I will try.
i read in the FEFF 8.4 manual, that the 6 modules are executed in ascending order. But now i'm confused concerning the modules 3 till 5. In module 3 the full multiple scattering is calculated. But only after this module the path enumeration (module 4) and the scattering amplitudes (module 5) are computed. I thought the path enumeration and the scattering amplitudes are required for calculation of the full multiple scattering? Why are these quantities calculated after the fms module? In my opinion this doesn't make sense.
You are not understanding things correctly. Full multiple scattering is one way of computing the XAS function. A path expansion is another way. FMS is exact (within the limitations imposed by the approximations made in the calculation, of course) in the sense that it computes all orders of scattering within the cluster in one swoop. Because the size of the angular momentum basis for the matrix algebra grows quickly with energy, there is a practical limit in energy beyond which FMS becomes computationally too expensive. Path enumeration is inexact in the sense that at low k, it can be shown that a path expansion is non-convergent. That is, near the edge, you cannot possibly sum up enough paths to converge on the result obtained by FMS. Fortunately the cost-per-path does not change with energy and the number of paths required is quite modest away from the edge. And, of course, the path expansion is useful for EXAFS analysis. Even if you were willing to invest the resources to compute FMS well above the edge, a path expansion would be more fruitful for interpreting that part of the spectrum. One strategy to computing the full spectrum (i.e. near the edge through hundreds or thousands of volts above the edge) would be to define a cross-over, using FMS before the cross-over and a path expansion afterwards. So, what does FMS need? FMS needs the potentials, which are computed in the prior modules. Scattering amplitudes and phase shiftfs also needs the potentials. So FMS needs modules 1 and 2. Scattering functions need modules 1, 2, and 4. Computing the XANES requires FMS or FMS+Scattering functions, depending on whether you are using the cross-over strategy.
Another question affects the paper (page 9 and 10) of Zabinsky et al (attached). In the section "path enumeration" (respectively one side bevor) he introduced the quantity r_tot = total path length and R = r_tot/2 = effective path length. My question is, what is the meaning of the effective path length? In my opinion r_tot is the way and the way back? And R < r_tot is the path length without negligible contributions (scattering amplitude,...), is this right?
I think you'll agree that, as long as we all agree on the ground rules for the conversation, it doesn't actually matter whether we use R_tot or R_tot/2. They don't differ in a significant way. If we agree and do the right thing at all stages of the calculation, everything will be cool. That said, R_tot/2 is a much better choice. For a single scattering path, R_tot/2 is the interatomic distance. The interatomic distance is often the deliverable quantity in an EXAFS analysis. So, for SS paths, R_tot/2 is the path of least confusion. For a MS path, R_tot/2 is the approximate position of the peak of the magnitude of the Fourier transform of the MS path. This is only true when doing the integral of Fourier transform of chi(k) over d(2k). That is the widespread convention in EXAFS. It would be confusing, then, if the language of XANES treated distance differently from the language of EXAFS. Conclusion: effective path length = yay!
The next question concerns the XANES-Card. What is the meaning of k respectively xkmax? In my opinion k is the wavenumber (of the photo electron wave?) which occurs in the xmu.dat in the third column, isn't it?
According to the document (which is often a useful thing to consult) http://leonardo.phys.washington.edu/feff/wiki/index.php?title=XANES xkmax is the maximum k value of the XANES calculation. That is, the FMS calculation will be made out to the energy corresponding to k=xkmax. HTH, 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/