[Ifeffit] a question about the white line

[Kelly, Shelly D.]

> -----Original Message-----
> From: Newville, Matthew G.
> Sent: Tuesday, December 07, 2004 11:17 PM
> To: XAFS Analysis using Ifeffit
> Subject: RE: [Ifeffit] a question about the white line
> 
> Shelly,
> 
> That seems like a lot of steps to me, and it almost seems like
> you're mimicking by hand the use of a standard chi(k) with
> autobk/spline().  Do you find very different results using a
> standard chi(k) with autobk/spline()?  If so, do you have any
> suggestions for how the more automated version could be improved?
> 

[Kelly, Shelly D.] 
Hi Matt,

You are right. My method does use a lot of steps.  I have seen "large"
differences in the background removal for the first two full cycles in
k-space.   I will show a figure, soon.  But often what happens is the
default spline cuts the EXAFS data too many times through the edge.  

Your suggestion for an automated approach is a good one.  Here is an
idea.

(1) Set kmin high in the spline range, so that you do not use the first
two oscillations in k.  
(2) Fit the first shell, in a way that makes sense.  
(3) Use the theory to remove the background using kmin=0 in the spline
range.  As Matt points out you can tell if the theory is reasonable by
the shape through the white line region.

You will get to the same place as before.  -Assuming that you use a
reasonable theory and your original choice of Ezero is not too far off.

I will try it out on some tough data and let you know how it goes.

Shelly