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@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@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@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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