[Ifeffit] ODP: problem with E0 (enot) parameters

Zajac, Dariusz A. dariusz.zajac at desy.de
Sat Jun 20 15:38:44 CDT 2009

Dear Scott, dear Arthur, dear Anatoly, dear Josh, dear Matt and dears All,
thank a lot for you involement in my problem.
I would like once more to appologize for that stupid idea to post message with large attachment, once I know I shouldn't  do it, second I should check how large the files are... I posted the message just before I left home and when I checked it home was too late - I couldn't cancel my post...
thanks once more.

Dear Matt,
answering to your post:
ad 1 - I have fitted up to 6A just to see the fit, the resonable fot can be only for first 2 peaks for C and N. Adding next parameters do not change the E0. the free parameters I had are only C and N, rest is combination of these 2 + extra for H, O, K at higher R. I have to check once more how the fit is without background.
ad 2 - I have no problem with distinguishing C and N - the sample is quite well ordered - cyano bridges. Only the multipath scaterring between C and N gives the contribution to first 2 peaks, but it is not possible to have good fit without them, thus I think the fit I have obtained is resonable even large E0. I do not observe flipping of atoms. 
ad 3 - yap, I havent worked with W, thus I asked you.
once more thanks for teh help and good advices


-----Wiadomość oryginalna-----
Od: ifeffit-bounces at millenia.cars.aps.anl.gov w imieniu Matt Newville
Wysłano: So 2009-06-20 17:40
Do: XAFS Analysis using Ifeffit
Temat: Re: [Ifeffit] problem with E0 (enot) parameters
Hi Dariusz,

Just to follow up on Stefan's request to not post large attachments to email:

The ifeffit web page is a wiki, and would be an ideal place for
posting the data (feff inputs, project files, images of plots) for
such questions.  You can read up on the wiki page for how to do create
pages and post attachments.   I'd encourage anyone with similar
impulses to post large files to use the wiki for such discussions.

To follow up on the rest of your questions and this thread:

I agree with Jeremy's advice / observation that having an E0 shift
push E0 "over the white line"  is not so unusual when using Feff6,
especially for L3 edges.   I'm not sure there's a good reason for it
besides "Feff6 makes approximations".

I'd also add a few other points:
  1.  In your original post, you had a huge number (30!) of variables.
 I'd recommend that you start much more simply than that.  In
particular, I'd suggest starting without refining the background.
  2.  As you know, It's hard for EXAFS to distinguish between C, N,
and O backscatters, and so "flipping atoms" around to positions that
are "obviously incorrect but consistent with the EXAFS data" is
something you'll need to look out for, especially with so many
variables (see point 1).
  3.  My understanding is that W L3 is particularly hard.  If you
haven't already done so, I'd suggest checking the literature on this
and doing as many sanity checks as possible.

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