[Ifeffit] The dependence of So2 on neighboring atoms

Bruce Ravel bravel at bnl.gov
Fri May 1 09:08:54 CDT 2009


You may find this recent paper by John Rehr and his team useful.  It
outlines recent progress in Feff in computing intrinsic losses with an
multipole model.  Errors in intrinsic losses are among the many "sins"
that Matt refers to and so is related to the evaluation of S02 in a fit.



> > It is known that So2 is the relaxing factor of passive electron.
> > So I think that So2 is affected by neighboring atoms because the
> > potential is made by those
> > Especially, If neighboring atoms are heavy atoms, the contribution will
> > increase. Is it right?
> > If it is right, what is the dependence of So2 on neighbor atoms?
> Usually S02 is separated from other loss terms as the "passive
> electron" factor, to account for the relaxation of the other Z-1
> electrons to the sudden presence of that 1 core hole that the
> absorption process created.  In this view, it is purely an effect of
> the absorbing atom, and is not affected by the neighboring atoms.
> In a quantum mechanical view of Fermi's Golden rule, absorption is a
> transition of an atom from an initial (ground) state of an atom to a
> final (excited) state of the atom.  Most of the change is the presence
> of that core hole (which gives the edge jump), a lesser part of the
> change comes from the created photo-electron interfering with the
> neighbor atoms (which gives the EXAFS), and a smaller part yet comes
> from the Z-1 electrons of the atom relaxing to the presence of the
> core hole (which gives S02).
> You are right that the highest energy-level electrons of atom (ie,
> those least bound to the atom), will interact with neighboring atoms,
> and therefore change the potential.  From the point of view of deep
> core levels, the chemical bonding of the valence-level electrons is a
> pretty small perturbation and is generally ignored when considering
> S02.
> Of course, for analysis, the S02 parameter is more or less a fudge
> factor covering up some sins in the measurement and other sins in the
> approximations in the calculations for the loss terms.
> --Matt
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 Bruce Ravel  ------------------------------------ bravel at 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/

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