[Ifeffit] Pb compound determination fitting issue

[Bruce Ravel]

On 02/25/2015 05:20 AM, Pieter Tack wrote:
>
> I recently acquired an EXAFS data set (albeit somewhat noisy) on the
> Pb-L3 edge. I have normalized it using Athena, and tried to fit the
> first shell using Artemis. I attached the Athena project file as well as
> the Artemis project file of what I obtained thus far.
>
> We have reason to believe the unknown compound is PbO or PbS (or a
> mixture). However, when I look at the R space curve, the first
> coordination shell is at rather low path distance (1.7 A), whereas one
> would expect larger bond distances for PbO and PbS. I found that a
> similar bond distance as to what the experimental spectrum shows can be
> found in PbCO3, but a fit using the mineral data of PbCO3 also doesn't
> do the trick.

Hi Pieter,

I am willing to believe that the sample might contain a mixture of O
and S.  The first big peak reasonably looks like it has scatterers at
two different lengths -- a big peak at about 1.5 and a shoulder at
about 1.9, plausible distances for those scatterers.

That said, your data isn't all that great.  As Fred points out, Pb is
never a spectroscopist's favorite absorber.  Your data are quite noisy,
both in terms of statistical noise and systematic error.  The level of
shot noise is rather high, as you can see from the "fuzziness" of the
data beyond about 6 A^-1 and in the rather high level of the high-R
portion of chi(R).

The feature at about 10 A^-1 does not look at all like EXAFS to my eye.
In fact, I suspect that you have a bit of bismuth contaminating your
sample.  The feature at 10^-1 A looks kind of like a small edge step.
Translated into energy, that feature is at about 13420 eV, which is
the Bi L3 edge energy.  Your Athena project file had kmax for the
Fourier transform set to 10.5.  That is certainly a mistake.  If I am
correct about the Bi contamination (if we were discussing this at my
beamline, I wouldn't be so circumspect, but circumspection is
appropriate for the mailing list!), you certainly must stop the FT
before 10 A^-1.

I am also troubled that you had the kmin for the FT in the Athena
project file set to 0.1 A^-1.  That is a poor idea.  By its nature, the
background removal is most uncertain near the edge, where the data are
changing most dramatically.  The background function is a piecewise
polynomial -- it simply does not have the freedom to follow the
swiftly changing data in that energy range.

Changing the FT range to [2.5:9.5] yields the chi(R) spectrum in the
attached image.  This does not leave much information content -- a bit
more than 5 independent points, given the R range of [1:2.2] that you
selected.  You will be hard pressed to do substantive analysis on
these data.  A sensible fitting model for a situation with two kinds
of scatterers would include a S02, an E0, and deltaR and sigma^2 for
each scatterer.  That's 6 parameters, which exceeds the information
content of these data.

Without better data, reliable over a longer range (or some kind of
multiple data set analysis, or some kind of constraint on the model),
you are not going to be able to meet your needs.  This one data set
simply does not support the amount of information you need to extract.


> In any case, no matter what I try to fit in Artemis, nothing seems
> decent. Is the EXAFS fitting simply impossible due to the noisy data,
> did I extract my Chi(k) wrong or am I encountering some other issue?

My executive summary is that you likely have a Bi contaminant in your
sample (probably not your fault, it was probably in whatever you used
to prepare the sample).  This limits your usable data range, which
severely limits the information content of the data.

Other than a few technical issues (mostly choice of FT range) you seem
to be thinking about the problem correctly.  Your best bet is to
figure out how to make a better measurement at you next visit to the
beamline.

HTH,
B


-- 
  Bruce Ravel  ------------------------------------ bravel at bnl.gov

  National Institute of Standards and Technology
  Synchrotron Science Group at NSLS-II
  Building 535A
  Upton NY, 11973

  Homepage:    http://bruceravel.github.io/home/
  Software:    https://github.com/bruceravel
  Demeter:     http://bruceravel.github.io/demeter/
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