[Ifeffit] LCF with different scales

bruceravel1 at gmail.com bruceravel1 at gmail.com
Sat Sep 3 07:56:02 CDT 2011

Hi Hana,

 > Hi list,
 > I'm looking for a way to make LCF with standards that were measured at
 > different occasions and beam-lines. I did read previous comments regarding
 > this issue, but still need to clarify:

There is no a priori reason why you cannot do so.  Two beamlines that
use the same mono crystal should measure the same data on the same
sample, within measurement uncertainty, of course.

The problem is that differences betwee the beamlines (differences in
resolution, differences in detector linearity, different gremlin
species mucking up the works, etc) result in systematic uncertainty in
your results.  As long as you are mindful of this systematic
uncertainty and honest about reporting it, there is no reason that LCF
cannot be done effectively.

 > 1. Does limiting the range to the smallest range help in this case
 > (if spectra were measured with the same steps size, and a
 > calibration standard was used)?

Well, putting the data on a common energy grid is certainly
essential.  If you cannot do so (using a calibration standard or
otherwise), then you cannot do LCF.

As for limiting the data range, I am not clear how that helps.  If the
problem is different resolutions, then any data range of any size that
includes the edge will be effected.

I suppose that if one of the beamline has a horrible problem with
runout error, then limiting the data range might help.  But does
anyone's beamline really have that problem?

 > 2. For harder situation; when different steps were used - I found
 > that sixpack (according to its manual) interpolating all spectra to
 > the same energy grid - does this solve the problem?

I can't comment well on Sixpack, but I will say that Athena always
interpolates regardless of the provenance of the data.  It is unclear
to me how the fit could be constructed in a numerical sense if the
data are not interpolated onto the same grid.


 Bruce Ravel ------------ bruceravel1 at gmail.com
 Homepage --------------- http://xafs.org/BruceRavel
 EXAFS software --------- http://github.com/bruceravel/demeter/

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