Another question to make sure. Since I am new in my group, at the beginning I was told to do the energy calibration by comparison of the spectra of MoS2 collected from different synchrotrons, just like the way described in the first email (first align MoS2 from DESY and ERSF, then align other samples from ERSF with MoS2 from ERSF to perform energy calibration). Is this processing reliable or reasonable? Because we are having some disagreement in our group now. So your opinion will be important. Thanks. TUM Lei Sent from my iPhone

On 14 May 2014, at 22:43, Matthew Marcus wrote:

Which edge is this for? I'm guessing MoK since you talk about using a foil for calibration.

Could the problem be polarization? Since MoS2 is a highly anisotropic material, you would expect strong dichroism in both XANES and EXAFS regions.

It's possible that the resolutions at the two beamlines are different, so that a weak pre-peak might not show on the worse one.

You said something about putting the foil in the sample position. Was the data taken in transmission? If in fluorescence, then it will be subject to over-absorption and the derivative will look shifted to the left. If in transmission, and you find that the calibration is stable, then I would think that it should be OK. I work on a beamline with no third foil and it works well. Another way to check is to inspect I0 for monochromator glitches and see if they move. You can use them as internal calibrants.

Whatever you do, note that different people use different reference values for edge energies, so whenever showing XANES or quoting energies, you should always make clear that, for instance, the Mo K-edge for Mo foil was taken to be 20000.0eV (or whatever you use). mam

...

On 5/14/2014 1:23 PM, Lei Zhong wrote: Dear all,

we had beam line time in DESY (Hamburg) and ERSF (Grenoble) for NiMo sulfided supported samples. In DESY we got quite nice foil reference but in ERSF we failed since the third chamber showed problems. Therefore, we have critical issues to do the energy calibration for spectra collected from ERSF. We collected spectra for MoS2 crystals. So we are thinking about if it is reasonable to compare the spectra of MoS2 for the energy shift and further apply the energy calibration. Would you please give me suggestions? Thank you in advance.

Q1. In two beam lines, for exactly the same MoS2 crystal, we got difference in XANES region. Spectrum from DESY has a weak preedge 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?

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?

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?

Sorry for the long email. Thank you for the time reading it. Hopefully I made the questions understandable to you. Looking forward to your answer!

Thanks.

Regards, Lei Technical University of Munich

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Thank you so much for your generous sharing. Have a nice weekend. Best Regards, Lei 2014-05-15 9:36 GMT+02:00 Edmund Welter :

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

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