[Ifeffit] Case of "nonequivalent multiple atomic sites of absorbing atoms"
SCalvin at slc.edu
Wed Jul 28 07:05:23 CDT 2010
Currently, S02 is usually described as being due to the relaxation of
the other electrons in an atom when a core electron is removed,
resulting in incomplete overlap of initial and final states. This
appears to be a fairly good description, as careful experiments show
good agreement with theoretical calculations based on this idea.
Note, however, that there could be some other contributors to S02. A
photon could, in addition to exciting the core electron at the edge,
also excite a valence electron.
There's a small thread on the transferability of S02 here:
E0 is a tricky concept, in my opinion: it is the energy origin in the
EXAFS equation. Perhaps a theorist can give me a pithy physical
interpretation of what happens at that energy, but I don't know there
needs to be anything; at k near 0, the path expansion is not
convergent, so I'm not sure we should expect anything special to
happen exactly at 0. In other words, it's not exactly the Fermi level
or any other special energy.
E0 is dependent on oxidation state; it can shift by an electron volt
or two when oxidation states vary.
Note that oxidation state is a simplistic measure of what's happening
with the electron distribution in a material. Suppose fluorine is
substituted for iodine in some material. Formally, the oxidation state
of the atom they are bonded to is not changed by the substitution. But
in reality, the electron distribution is different, and a small E0
shift would not be surprising.
I think the bottom line, then, is this:
S02 is completely transferable for the same element at multiple
Delta E0 is transferable with some caution for the same element at
multiple absorbing sites if the oxidation state is the same.
One other note: there's no rule that when trying constraints, you have
to start unconstrained and add constraints to see the effect on the
fit. With complicated systems like yours, it often pays to start with
unrealistically simple constrains (not only E0's and S02's the same,
but also sigma2's), and see if you're on the right track. Then look at
the effect of relaxing constraints.
Sarah Lawrence College
On Jul 28, 2010, at 4:00 AM, Jatinkumar Rana wrote:
> Dear Users,
> Since long, i was trying to understand the physical meaning of term
> "Delta E0" and "S02" in EXAFS equation. I have little bit of idea
> about both of them. for example, S02 is element specific and it is
> transferable between samples (if we consider same absorbing atom).
> However, I am not able to realize their importance in terms of their
> "physical meaning" as far as interaction of photoelectron is
> concerned. Therefore, it is difficult for me to understand their
> influence on EXAFS.
> I am dealing with a case of "nonequivalent multiple atomic sites of
> absorbing atoms". It is quite obvious that in such kind of case no.
> of variables are more than no. of independent points and there is a
> need to constrain the parameters to solve such problems.
> I have following questions :
> How do i understand "Delta E0" and "S02" theoretically (in terms of
> photoelectron interaction) ?
> Can i constrain "Delta E0" for all absorbing atomic site as same ?
> (my assumption : all absorbing atoms are at same oxidation level)
> your comments and suggestions would be highly appreciated...
> Best regards,
> Ifeffit mailing list
> Ifeffit at millenia.cars.aps.anl.gov
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