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
Edmund