[Ifeffit] Athena (linear combination fitting)

[Bruce Ravel]

On Thursday, May 12, 2011 03:09:12 am Dilnesa, Belay wrote:
> Dear Dr. Ravel,
> Thanks so much,for information you gave me. I subscribed already in the
> mailing list. I have a complex system to fit. I have a complex spectra
> from a hetrogenous sample and a number of reference samples. I am doing
> EXAFS LCF. To choose from the combinations after fitting is diffcult
> because the R factor of the first 5 combinations with diffrent reference
> combinations is similar. And as I see from my fitting the alignement plays
> really an important role  on the LCF. You know when I made allign marked
> groups based on the standard, it alligns well the enregy but when I see
> the k-plot, it shifts. And my fitting space is on chi(k) which is
> sensitive when I allign. It is complex system. The procedures I am trying
> are, Normalization-allignement-LCF (this procedure alligns the energy well
> but the chi shifts and this affects my fitting because I am fitting on chi
> space. The other procedure i checked, Normalization-calibration-LCF (this
> alligns the energy good but for some spectra it ! shifts again). When I
> allign it manually by excel sheet, it works good. Can I use the following
> procedure...normalization, calibration-allignemet-LCF? I just found a well
> alligned spectra in E and K plot from this procedure.   I will be happy if
> you comment on those procedure. Thanks so much for the keen help!

Hi Belay,

LCF fitting to chi(k) spectra is sensitive to choice of E0.  This can
be quite challenging in a system with large E0 shifts between species.
That is, if the oxide and the zero-valent metal are both present in
the unknown data, then dealing with the low-k portion of the data can
be tricky for a number of reasons.  You only get to do a single
background removal of the unknown data (obviously!) and how to do the
background removals for the pure-phase standards in a way that is
consistent with the background removal of the unknown is non-obvious.

For this reason, it is a good idea to begin the LCF fit at a fairly
large k value -- something like 3 or 4 -- to avoid the systematic
uncertainties associated with the background removals.

That said, I think your procedure is reasonable, although I am bit
unclear what you are doing in your "calibration" step.  Are you
setting all your data and standards to have the same E0?  For LCF
analysis, that is probably a reasonable way of dealing with the
background removals in the presence of valence edge shifts.

However, it is hard for me to comment more clearly without seeing your
data.

BUT!  There is a much better way of convincing yourself that your
analysis is being done in a defensible manner than asking the opinion
of an "expert".  Do an experiment!

As I recall from your earlier email, you are working on iron
compounds.  OK.  Prepare a series of artificial samples containing the
Fe standards that you are using to interpret your real iron data.  Mix
the standards together to form a set of combinations.  Go to the
synchrotron and measure spectra on the mixtures and apply your
analysis procedure.  If you get the right answers, then you might
reasonably have confidence in your analysis procedure.  If you get the
wrong answer, then you can reassonably question either your procedure
or the quality of the software that some clown wrote.

HTH,
B



-- 

 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/