[Ifeffit] Ifeffit Digest, Vol 67, Issue 11

Matt Newville newville at cars.uchicago.edu
Tue Sep 9 10:28:10 CDT 2008


> Same problem I can address in a different way.
> Suppose the energy is not linear in case of a Pt-L3 scan.
> We can set Pt-L3 edge at known 11564 eV. Scanning all the way to
> L2 edge and say we find it 200 eV off (exaggerating), can we set
> in Athena the L2 edge at known 13273 eV, while the L3 edge is still
> at 11564 eV?

You could, in principle, but it's not a good idea.  If the energy is
"not linear", then Athena is not going to be able to help you, as it
has no way of knowing the details of the non-linearity.   The energy
shift in Athena is simply a constant energy shift

    E_new(i) = E_old(i) - Constant_Offset   for all energy points i

For data that has nearly-correct energy values, this constant shift is
a reasonable approximation to the proper correction. Importantly, it
can be done without knowing any details of how the energies were
defined.  That is, you don't need to assume that the data is from a
rotating double-crystal monochromator.  Even if it is a double-crystal
monochromator, you do not need to know the monochromator crystal cut
or the angular precision of the rotation stage.

To do the energy shifting correction more properly, or to correct very
badly calibrated data, you do need to know the details of how the
energies are defined in the first place.  For data with energies so
far out of calibration that the Pt L3 and L2 edge would drift by 200
eV, there is probably a problem with how the energies are set.  For a
double-crystal monochromator, this could be an incorrect lattice
constant,  an angular offset problem (ie, at which rotation angle are
the lattice planes parallel to the incident beam),  or a
wobble/run-out in the rotation axis.  For more details, see R. F.
Pettifer and C. Hermes, J Applied Crystallography 18, p 404 (1985) and
 J. L. Glover and C. T. Chantler,  Measurement Science and Technology
18 p 2916 (2007).   For data collected in quick-XAFS or
dispersive-XAFS modes, the details of how the energies are set in the
measurement are likely to be quite different.

In general, Ifeffit is capable of allowing you to make such
corrections (ie, it has trig and algebraic function so that you can
put the recorded energies back to angular steps, correct those if
needed, and then recast to energies), much in the same way as it can
be used to convert data from the Lytle database (which generally
records monochromator steps, not energy in eV) into usable data.   But
again, and as many people have echoed, if the beamline cannot provide
reasonably correct energy values, then Athena cannot do this for you.



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