On Tuesday, April 12, 2011 03:24:22 am goodhei8@gmail.com wrote:
1, FEFF takes Z atoms at equal distance as same. It does not care some have neighbor A, some have neighbor B. Should I worry about it? and when?
In Feff6, two atoms that are at the same distance and have the same unique potential make the same contribution to the EXAFS. I don't quite understand what you want to worry about.
2, Does FEFF consider about valence state (like Ur 3 or Ur5 in center)?
Feff6 does not understand valence. This is one of the differences between Feff6 and Feff8. Feff8 uses a self-consistency loop to transfer charge between the atom types, resulting in something that can be related to formal valence in some situations. The follow-up question you have to ask is whether EXAFS analysis is actually sensitive to charge transfer. In the regime in which the EXAFS fit is evaluated, the photoelectron has large kinetic energy and is not particularly sensitive to the details of the potential surface. In almost all situations, Feff8 is a modest improvement at best over Feff6 for EXAFS analysis. That is why we continue to distribute and use Feff6 with Ifeffit and Artemis. Of course, charge transfer is a dramatic effect in the XANES. But you were asking a question about EXAFS analysis. 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/