[Ifeffit] Ifeffit Digest, Vol 67, Issue 15

[Khalid, Syed]

Matt,

We do use a cam when doing QEXAFS. Along with time and
detector values we record the Bragg angle of the Si(111)
monochromator as a function of optical encoder readout.
The encoder values are random numbers but linear with the
Bragg angle. Under normal routine QEXAFS we have developed programs
to convert the QEAXFS data and make it compatible with Athena
analysis. That works fine.

In this particular case I wanted to compare Ni EXAFS scan done in conventional way to the one I got
in 100 ms. For this particular scan I could not use the programs that we
developed (for data conversion to Athena compatible, for many reasons).

I wanted to see how a time axis converted to energy, compares with 
conventional EXAFS at FFT stage.

It was also to find out, as you mentioned, if two energy points will match,
then will it compare to conventional EXAFS in this particular case??

Syed Khalid

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Subject: Ifeffit Digest, Vol 67, Issue 15


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Today's Topics:

   1. Re: Ifeffit Digest, Vol 67, Issue 12 (Matt Newville)


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Message: 1
Date: Tue, 9 Sep 2008 11:14:12 -0500
From: "Matt Newville" <newville at cars.uchicago.edu>
Subject: Re: [Ifeffit] Ifeffit Digest, Vol 67, Issue 12
To: "XAFS Analysis using Ifeffit" <ifeffit at millenia.cars.aps.anl.gov>
Message-ID:
	<b8522e3d0809090914o2e3d6773n370158b56f370077 at mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1

Syed, Adam,

I think the suggestion from Adam and one of the suggestions from
Anatoly assume the energy linear in time.  Is that the really case?
For quick-scanning modes, many beamlines run the mono at a constant
angular velocity, so that time maps linearly to angle, which then
needs to be converted (non-linearly) to energy.  Then again, some
quick-scanning beamlines use a special cam drive, in which neither
angle nor energy would be linear in time.

Again, you need know these details in order to correctly assign
energies to the data.  None of it is particularly difficult, so
there's no point in guessing, and stretching or massaging the energy
scale until two energy points correct does not mean the rest are
correct.    You need to find out from the beamline how the data mapped
onto energy and fix that.   While you can probably use Ifeffit macros
within Athena to do this correction, this is really a job for the
beamline, not for Athena.  Having an option in Athena that allowed the
user to say "I don't know why my energy scale is bad, but can you
please stretch the energies so that these two energies are what I'd
like them to be" is a horrible idea.

--Matt


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End of Ifeffit Digest, Vol 67, Issue 15
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