[Ifeffit] Fitting Problems via Artemis

[Bruce Ravel]

On Friday 24 September 2004 05:32 pm, Carlo U. Segre wrote:
> Norbert answered the last two but I think that I can give a bit of an
> answer to the first one.
>
> On Fri, 24 Sep 2004, hsnuboy at ksu.edu wrote:
> > 1. What's the meaning and purpose of "Fit Background" in Artemis? I know
> > the backgound as "bare atom" in EXAFS theory. So, what kind of
> > situation we need to use this function?
>
> The background fitting in Artemis is a sort of second chance to remove
> background that was not completely removed in the initial processing
> (Athena).  It attempts to fit a background function to the residual below
> the lower R fitting limit (for R-space fitting).  I like it because it
> allows you to be somewhat less concerned about the initial data processing
> (it is still a good idea to be careful and thorough), particularly with
> difficult samples with noisy data.

What Carlo says is completely correct and is the reason I tend to use that 
feature as well.

It is important to understand why it works and why it is an OK thing to do.  
Let's first go back to Athena and the initial back ground removal.  In Athena 
the most important background removal parameter is usually Rbkg.  That 
parameter marks the cutoff in the space of the Fourier transform between the 
background and the chi(k).  To say that another way, when we choose a value 
of Rbkg, we are stating that we believe that the Fourier components below 
Rbkg are the Fourier components associated with the background and the 
components above Rbkg are the components associated with the chi(k).  The 
Autok algorithm then (essentially) tries to remove the background components 
while leaving the chi(k) components.

Doing this background then, in principle, leaves a spectrum that only has the 
Fourier components associated with the data and none of the components 
associated with the background.  In practice, this rarely happens -- 
particularly in the case of materials with white lines.  Pretty typical is to 
have a certain amount of spectral weight below your original Rbkg.

Thus Artemis has the option of doing an additional refinement of the 
background.  Between Rmin and Rmax, the Feff calculation is used to fit those 
Fourier components.  Between 0 and Rmin, a spline is used to fit those 
Fourier components.  Thus Rmin acts rather like Rbkg in Athena.

The advantage of doing this is that it may reveal correlations between the 
background spline and the parameters that you are trying to fit.  It has the 
additional cosmetic advantage of often making the fit look pretty.

HTH,
B

-- 
 Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
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