[Ifeffit] a question about the white line

[Kelly, Shelly D.]

Hi Bruce,

I have worked with tough background removals.  They are particularly
difficult the first time you encounter each one.  My methodology goes
something like this.  It isn't easy and sometimes very frustrating.

1) Determine how broad the rise in the adsorption edge.  How many eV is
it from the bottom to the flat part above the edge.  If you edge is
broad > 10 eV then Ezero will be somewhere high on the adsorption edge.
If you edge is sharp then Ezero will be somewhere low on the adsorption
edge. 

2)  Remove the background. Look at the FT with all three k-weights.
Determine if Rbkg is reasonable.  

3)  You must have a good idea about the first shell.  Run Feff and look
at the theory with all variables set to zero, Ezero =0, sigma2=0 and
delr=0.  If you have a split shell be sure to include all the paths that
make up the first peak in the FT of the data.

4) Compare theory to data using a well padded k-range.  Leave out the
first two oscillations in k, starting around 4 to 5 inverse angstroms.
Set Ezero equal to zero.  Run a fit, determine delr for the paths and
sigma2.  Look at the fit and the data in k-space with all three
k-weights.  Check to see if delr and sigma2 are reasonable.  See if you
think that moving Ezero around in Athena will help line things up at low
k.  

5) Import the "fit" into Athena and use it as the theory.  Remove the
background moving Ezero to line up the data and the theory at low k.
And then start all over.  

6)  This procedure will not work if you use the wrong theory hence the
frustrating part.

HTH

- If someone on the list is willing to try this procedure out and gets
stuck and you are willing to share all.  I will continue to help on the
mailing list.  

Shelly


Shelly Kelly ---------------------------- skelly at anl.gov

Bldg 203, RM E113                           phone: (1) 630-252-7376
Argonne National Laboratory              fax:     (1) 630-252-9793
Argonne, IL 60439                             www.mesg.anl.gov 
              


> -----Original Message-----
> From: Bruce Ravel [mailto:ravel at phys.washington.edu]
> Sent: Tuesday, December 07, 2004 3:00 PM
> To: XAFS Analysis using Ifeffit
> Subject: Re: [Ifeffit] a question about the white line
> 
> On Tuesday 07 December 2004 03:47 pm, you wrote:
> > Hi Bruce,
> >
> > Feel free to have them contact me. I have done a lot of Re
compounds.
> >
> > Jeff
> 
> Hmmm.... The point of a mailing list is to have a public, searchable
> archive where useful information can be discussed and shared in an
> open manner.
> 
> I could have just answered the question myself, but I thought that it
> was one of broad interest.  Thus I had hoped to solicit discussion on
> the mailing list.
> 
> B
> 
> --
>  Bruce Ravel  -----------------------------------
> ravel at phys.washington.edu
>  Code 6134, Building 3, Room 405
>  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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