2007/10/9, Ravel, Bruce <bravel@bnl.gov>:

> Dear all,
> I´m trying to deal with exafs data of a Ni-phosphate compound, using
> Athena/Artemis. This is a simple structure (I think), with the
> hexacoordinated ion surrounded  by water molecules on the first shell. The
> question I have concerns the negative value for e0 I´m having.
> Is this a bad thing?
> Maybe I´m doing something wrong?
> I´ve tried some issues, as those proposed on the Shelly Kelly´s tutorial,
> but without success.
> Can you help me, please?
> Attached are an example of the Athena and Artemis projects i´m using.
> Thanks a lot.
> Mauricio

Hi Mauricio,

I think you might not be understanding the purpose of the e0 shift in
EXAFS analysis.  When we process data, we choose an edge energy which
sets the zero of wavenumber.  When we make a Feff calculation, it
somehow chooses its interstitial level, setting its zero of
wavenumber.  There is no reason to expect that these two wavenumber
grids will be aligned.  The purpose of the e0 shift in EXAFS analysis
is to align the k-grid of the data with the k-grid of the theory.
Just as you cannot expect those k-grids to be aligned a priori, you
don't know which way they will be misaligned.  It is just as likely
that you will need a negative value of e0 as a positive one.  As long
as you have not made any mistakes processing the data (such as a very
poor choice of the edge energy) or in making the theory (such as
computing for the K edge when you actually measured the L3 edge), then
the absolute value of the e0 parameter should be small -- a few volts
or less.

There is nothing obviously wrong with the e0 shift in the artemis
project you attached -- it was only about -2 volts.

HTH,
B

PS.  Sometimes you see fits with more than one e0 parameter.  That
might be justified if you have some reason to think that one component
of your fit is electronically dissimilar from the rest of your fit.
Typically, we use a single e0 parameter which then has the
interpretation I gave above.

--
Bruce Ravel  ----------------------------------- bravel@bnl.gov

My homepage:    http://cars9.uchicago.edu/~ravel
EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/

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