[Ifeffit] Problems in Energy calibration

Edmund Welter edmund.welter at desy.de
Thu May 15 02:36:02 CDT 2014

Dear Lei,

> Q1. In two beam lines, for exactly the same MoS2 crystal, we got
difference in XANES region. Spectrum from DESY has a weak pre-edge peak
resolved in XANES while ERSF doesn't. And the derivative shape of the
two spectra for MoS2 collected from two synchrotrons are not well
overlapped. Is it acceptable?

If you measured the Mo K-edge at 20000 eV at DESY you were most probably
using the 311 crystals (at least that is what I or one of my colleagues
here would have suggested) As Matthew already mentioned the differences
in the pre-edge region might be due to differences in the energy
resolution (for instance if you used 111 crystals at ESRF).

> Q2. I tried to overlay the first derivative of the two spectra of
MoS2. There is a energy shift at 2.5 eV from the different synchrotrons
if we take the one from DESY as standard. Can I apply the energy shift
at 2.5 eV for MoS2 from ERSF and then overlay the first derivative of
the other NiMo samples with MoS2 to get the energy calibration? Or
should I apply 2.5 eV for all the NiMo sulfided samples?

I am not sure if I understand exactly what you want to do. Anyway, if
you find a difference of 2.5 eV between the edge position measured on
the same sample at different beamlines it makes sense to define the
spectra from one beamline to be "correct" and shift all spectra from the
other beamline by the respective energy shift (here 2.5 eV). BUT (!) you
must be sure that the shift is  reproducible... If you measured
reference spectra you can easily check if this is true, you should not
observe any shift between the energy positions of you metal foil
spectra. If you have no reference spectra you could trust encoder values
(if you have them) or it becomes a religious thing and you just have to
believe (and maybe convince others to believe too...;-)

> Q3. Since we lost the signal for the foil reference in ERSF, we tried
to put the foil in the sample position. The first derivatives of the
foil spetra from time to time overlap quite well. Should we set the
energy calibration at 0 according to the stable foil signal?

So you checked the reproducibility of your energy axis and you can be
more or less sure that the energy axis did not shift during your
measurements. Fine, so there is no need to re-calibrate your one ESRF
spectrum with respect to another ESRF spectrum, but if you want to
compare them with the DESY spectra you still have to shift either all
the DESY spectra or all the ESRF spectra by 2.5 eV. And as Matthew
already mentioned you should document what you did so that someone else
can trace your energy positions back to a common standard (in your case
the metal foil and the usually used k-edge position of 20000.0 eV from
the orange book.

Best regards

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