[Ifeffit] Chi(E) data export and import

Bruce Ravel bravel at bnl.gov
Fri Apr 1 07:25:07 CDT 2016

On 04/01/2016 07:58 AM, Matteo Busi wrote:
> Now for my master thesis, I'm performing a correction to chi(E) in the E
> space regarding to some geometrical properties of the sample.
> I have also tried to re-import the corrected data as a chi(k), having
> reproduced the right k grid column, but it looks totally wrong.
> The thing I'm not sure about is if the chi(E) needs to be fourier
> trasnformed into chi(k) or not.
> That's because I would have liked to make comparison in the Athena
> software between the two chis, but if that's not possible I will keep it
> to the external software (mathematic) with which the correction is
> performed.
> Or if you can share me the fourier transform routine performed I can
> include it to my correction script.

You seem to be thinking about this wrongly.  The chi(E) that gets 
written to the mu(E) ascii file using the thing we discussed last time is

   mu(E) - bkg(E)

That's it.  The difference between the chi(E) in the mu(E) file and the 
normal chi(k) is that (mu(E)-bkg(E)) is interpolated onto a uniform 
k-grid.  There are at least two reasons for doing that, (1) every data 
sets then ends up on the same k-grid, which is a convenience, and (2) a 
uniform k-grid allows use of a /fast/ Fourier transform.

If you want to overplot chi(E) and chi(k), you also have to do the 
coordinate transform between energy and wave-number, which depends upon 
the value of E0.

You have never clearly explained what you are up to and why 
interpolating the normal chi(k) back onto the energy grid was 
inadequate, so I don't know what else I can say that would be helpful.


  Bruce Ravel  ------------------------------------ bravel at bnl.gov

  National Institute of Standards and Technology
  Synchrotron Science Group at NSLS-II
  Building 535A
  Upton NY, 11973

  Homepage:    http://bruceravel.github.io/home/
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