[Ifeffit] LCF on Larch - EXAFS Region
newville at cars.uchicago.edu
Mon Aug 13 00:19:23 CDT 2018
On Thu, Aug 9, 2018 at 10:04 PM Mike Massey <mmassey at gmail.com> wrote:
> This is interesting. Could you say more about your skepticism of the
> robustness of EXAFS LCF, Matt?
> To be fair, it suffers from many of the same drawbacks of XANES LCF, plus
> others. But I'm curious about your thoughts on it since yours seems to be
> what amounts to a "strong opinion" on the subject.
I would not say that no one should ever do linear combination fitting for
EXAFS. For sure, linear analysis of XANES is quite robust and verified
many times to give good results, at least at level of a few percent.
Linear analysis of EXAFS suffers more data processing challenges and
conceptual problems that limit its robustness. For sure, there are cases
for which it can work well.
Any linear analysis (LCF, PCA, MCR-ALS, etc) of XANES works reasonably well
(typically to a few percent) because:
a) the processing needed is minimal. Data need to have a common energy
calibration better than the intrinsic energy resolution -- typically energy
calibration of 0.25 eV or better will be OK. Data need to have a
consistent normalization of mu(E), typically to a few percent. Variations
in these processing steps will have a direct and negative effect on the
b) conceptually, the assumption is that there exists a nearly 1 to 1
correspondence between "local chemical configuration" and "measured XANES",
and that the "local chemical configurations" that are being investigated
are discrete and well-defined (ie "iron carbonate") and not continuous.
That is, if you determine that your Fe XANES spectra is "50% iron
carbonate and 50% iron sulfate" then implicit conclusion is that 50% of the
iron atoms are iron carbonate and 50 percent are iron sulfate, not that all
irons are 50% carbonate and 50% sulfate.
To be clear, linear analysis of XANES does not work well to ppm levels,
partly due to the poor experimental contrast (that is, mu(E) tend to all
look alike and features are intrinsically broadened to the ~eV level), but
also conceptually, because at the ppm level, local chemical configurations
are not always limited to 3 to 10 discrete states.
Linear Combination EXAFS is more challenging from both the processing and
conceptual point of view.
For Processing, EXAFS requires more data processing than XANES. The
selection of E0 and the background mu0(E) will have an effect on linear
analysis of EXAFS if not done consistently. It is not really obvious how
E0 or mu0(E) can be selected consistently for very different spectra.
Conceptually, EXAFS is much more sensitive to disorder and subtle
variations in the bond lengths (thermal or static disorder) and can have
significant variation in its sensitivity to second and further neighbors.
In that sense, EXAFS is much less discrete and much more continuous in its
variability across different kinds of local structures.
Again, this is not to say that linear analysis of EXAFS cannot ever work,
just that is probably more limited in applicability and absolute accuracy
than linear analysis of XANES. Of course, for EXAFS you can also do an
actual fit of structural parameters. The information content is somewhat
limited so that refining multiple overlapping components may not always be
possible, and linear combinations of end-member spectra may look
Hopefully, anyone who has other insights or experiences will be able to
correct any of my misunderstandings.
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