[Ifeffit] a question about background refinement

Bruce Ravel ravel at phys.washington.edu
Tue Apr 13 11:08:59 CDT 2004


Matt said:

> The thing that makes me the most uneasy is the relatively large
> values of Rbkg/Rmin (that is rbkg in spline() and rmin in
> feffit(do_bkg=1,....) which also refines the background) and how
> they change around in your analysis. Normally, you'd want to keep
> Rbkg/Rmin below the 'first shell peak', which is sometimes difficult
> to define for the asymmetric peak common with short metal-oxygen
> distances, but if the peak is around 1.3Ang, the Rbkg/Rmin of 1.5Ang
> seems like it might be too high.  Can you get decent results with
> a lower Rbkg/Rmin?  

I have a question for you.  Are you trying to determine the oxide
content?  That is, do you want to know what fraction of the Ru atoms
are in an oxide environment?  Or are you trying to account for the
oxide so that you better measure the metallic component?

If you don't need a good measurement of the oxide, then perhaps the
thing to do is to consider the oxide to be inseparable from the
background, and just let the spline to remove that oxide contribution
from the fit.

That's not as silly as it might sound.  As Matt explained, the purpose
of the spline is to remove components below Rbkg.  Another way of
saying that is that the components above Rbkg are the interesting ones
and the components below Rbkg are not part of your analysis problem.
One often use the AUTOBK spline to remove the step function, the
so-called AXAFS, and the low-frequency contributions due to
multi-electron excitations.  One removes all those things because they
are not the portion of the XAS spectrum that you want to fit.  All I
am suggesting is that you might lump the oxide in that category as
well, if you are only concerned about the metallic component.

Of course, you may find that the Ru-Ru path from the oxide contributes
in a frequency range similar to the metallic first shell.  But that's
another story.


 Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
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 Naval Research Laboratory                          phone: (1) 202 767 2268
 Washington DC 20375, USA                             fax: (1) 202 767 4642

 NRL Synchrotron Radiation Consortium (NRL-SRC)
 Beamlines X11a, X11b, X23b
 National Synchrotron Light Source
 Brookhaven National Laboratory, Upton, NY 11973

 My homepage:    http://feff.phys.washington.edu/~ravel 
 EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/

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