[Ifeffit] Athena (Question about LC fitting)

Yu-Ting Liu yliu21 at ncsu.edu
Sun May 15 12:45:16 CDT 2011


Hi,

Related to this issue, I have a question about whether alignment is
necessary for LCF when the oxidation state among sample and standards
is different. I used to align spectra of samples and standards before
LCF while handling P. However, I got confused about Se. For Se
speciation, I prepared six Se species with oxidation state of -2
(FeSe), 0 (elemental Se), 0 (seleno-cystine), 0 (seleno-methionine), 4
(selenite), and 6 (selenate) to fit samples collected from sediments
and biofilms. I am wondering whether the shift of spectra due to the
alignment would make standards mis-fit spectral features of samples.
Thank you so much for the help!

Yu-Ting Liu

Yu-Ting (Lusia) Liu
Postdoctoral Associate
Duke University
Civil and Environmental Engineering
Durham, NC, 27708-0287

-------------------------------------------------------------------------------
Belay,

I am CCing my response to the Ifeffit Mailing List, which is the
appropriate venue for questions about the use fo the software.  I
encourage you to subscribe to and use the mailing list.


On Tuesday, May 10, 2011 08:50:10 am you wrote:
> Dear Dr. Ravel,
> I am doing my PhD in title with identication of Fe-phases in cement
> hydrates. I am using Athena for LC fitting. I use EXAFS spectra for the
> fittings. My question is before LC fitting of EXAFS spectra , do I need to
> calibrate the energy? I have normlized and alligned the spectra very well.
> The problem is data calibration, when I calibrate the spectra,  the energy
> shifts and it destroys the fitting. Can you please give a clue why this
> happens or do I make a mistake on the procedure? Thanks so much for your
> cooperation.

Alignment puts a group of spectra on the same relative energy scale.
It is often necessary to do an explicit alignment when working at a
beamline with an unstable and unencoded monochromator, as might be
found at some older beamlines.

Calibration intends to put a single spectrum on the correct absolute
energy scale.  That is, if you know what point in a spectrum
represents a particular absolute energy, then calibration does as I
said.

LCF requires that the ensemble of data and standards all be aligned --
i.e. put on the same relative energy axis.  Once that is done, you can
proceed with LCF analysis.

LCF does not require that the data be calibrated, only that they be
aligned.  That is, LCF requires that all the data+standards be on the
same relative energy axis, but does not require that that energy axis
be calibrated.  The reason that is so is because LCF fitting is an
abstract, numerical function that treats one data set (the "data") as
a linear combination of two or more other data sets (the "standards").
No part of that mathematical process requires knowledge of the species
of the absorber atom.  Indeed, LCF does not require that the x-axis
represent energy, only that it be the same for all data+standards.

That said, the interpretation of your data might be easier to consider
and easier to present in publcation if your data is calibrated.  For
instance, the Xray Data Booklet tells us that copper metal has an edge
at 8979 eV.  Calibration is the process of transforming your data such
that its edge does, in fact, happen at 8979 eV.  In Athena, using the
calibration dialog both does an energy shift and sets the E0 value to
8979.

If you want to calibrate your data as well as aligning your data, I
find it less confusing to do the calibration first.  I believe that
the procedures can be commutative, but calibrating after aligning is
confusing to me and more prone to error for me.  Someone else might
give you different advice.

Finally, remember that calibration requires actual knowledge (as
opposed to some assumption) about what point in a spectrum represents
a particular absolute energy value.  While you might reasonably assign
the first big peak in the first derivative of copper metal to 8979 eV,
you cannot correctly assign the first big peak in the first derivative
of copper oxide to 8979 eV due to the upwards shift in threshold
energy in the oxide.

HTH,
B



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