Re: [Ifeffit] Ifeffit Digest, Vol 67, Issue 11
Anatoly: I will try your approach:
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?
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Subject: Ifeffit Digest, Vol 67, Issue 11
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Today's Topics:
1. FW: enrgy-axis-expansion (Khalid, Syed)
2. Re: enrgy-axis-expansion (Bruce Ravel)
3. Re: enrgy-axis-expansion (Anatoly Frenkel)
4. Re: enrgy-axis-expansion (Bruce Ravel)
5. ODP: enrgy-axis-expansion (Zajac, Dariusz)
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Message: 1
Date: Mon, 8 Sep 2008 13:46:47 -0400
From: "Khalid, Syed"
Is it possible in Athena to change the y-axis (energy scale) at two different energy points of an EXAFS scan?
I have a Ni foil spectrum as a function of time. The edge can be set at 8333.0 eV, and I want to to assign higher energy to a known feature in the Ni scan.
I am not 100% certain I understand the question, but I think the
question is about the calibration dialog. That is, you want to assign
an energy shift, but not referenced to the edge energy position.
Why can't you just set the energy shift by hand on Athena's main page?
B
--
Bruce Ravel ------------------------------------ bravel@bnl.gov
National Institute of Standards and Technology
Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
Building 535A
Upton NY, 11973
My homepage: http://xafs.org/BruceRavel
EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/
------------------------------
Message: 3
Date: Mon, 8 Sep 2008 13:59:42 -0400
From: "Anatoly Frenkel"
Syed,
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. Cheers, --Matt
participants (2)
-
Khalid, Syed -
Matt Newville