[Ifeffit] E0 on linear combination fitting: fit or not?

[Matt Newville]

Hi Pamela,


On Thu, May 11, 2017 at 12:19 PM, Carrillo Sanchez, Pamela <
pcarrillo at bnl.gov> wrote:

>
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> Hi all,
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> I have gone through the mail archives as well as following the
> instructions of Athena user’s manual as well as Scott Calvin’s book but
> still I find myself with the doubt of how correctly “choose E0”. I have
> aligned the data and standards with the merge of reference foils
> measurements of the metal (Mn). We did not have a reference foil placed
> while measuring the data so I used the merge of the references scans when
> the standards were measured and they showed a consistent energy shift.
>
>  I have set the same E0 for all the data and standard to the same E0, I
> chose the first peak of the derivative in the data (6MnRh_RED). When I do
> the LCF analysis without ‘fitting E0’ ,  I get a worse fitting than when I
> choose the fit E0 on Athena as well as different types of components on the
> fitting. Which one is the best choice for it ? From what I have read I
> think that without fitting E0 would be the proper one regardless of the
> “worse fit”.
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>


The recommendation to not fit E0 in linear combination fitting is generally
preferred, but also assumes that you have all data calibrated to the same
energy scale.  The concern for fitting E0 in such a linear analysis is that
energy shifts are often a sign of real chemical effects, not an indication
of a change in energy calibration.

But, you can definitely use Athena's linear combination fitting to figure
out what the energy shift is between two spectra on the same material
measured with different calibrations.

Generally, it's best to measure several standards at the same time, so as
to be able to assert that their energy scales (and resolution) are
consistent.   It's OK to measure some samples / standards at different
beamlines or different sessions at the same beamline as long you're careful
about calibrating energy consistently (say with the same sample such as
metal foil or hard-to-alter metal oxide) and the resolutions are similar.
These days most beamlines are pretty stable in energy calibration and
resolution.

If the standards are well-aligned internally, applying very different E0s
to the standards is probably not what you want.   Your "fit E0" example
shows E0 shifts varying between ~0 and -6 eV for the different standards.
I think that's kind of large - it's possible the energy scales for the
standards vary by that much, but you might want to double-check that too.

But, if you don't have calibrated standards, I might suggest asking the
beamline scientist if they have an idea what kinds of energy shifts they
expect.


Hope that helps,

--Matt
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