[Ifeffit] E0 correction in EXAFS fitting

[Matt Newville]

Dear Wojciech, 

It seems like this question has been asked many times, possibly in 
slightly different form.

Feff's choice of E0 can be off by several eV. Also, Feff is not
good at distinguishing formal oxidation state.  So believing
Feff's E0 may not be the best idea.  In my experience, Feff tends
to pick an E0 that is relatively far up the main absorption edge,
generally several eV above the maximum of the first derivative.  
That's consistent with the traditional lore that says the max of
the first derivative is the Fermi level, not E0 (lowest unoccupied
orbital).

If athena/autobk pick E0 to be on a sharp pre-edge peak, you
probably want to move it.  But the best placement for E0 is not
easy to make when there is a large pre-edge peak.

One camp says that since there is absorption, then it's at E_0,
and the pre-edge peak is really the absorption edge, followed by a
large negative EXAFS 'black line' oscillation that nearly sends
mu(E) to zero. The other camp invokes localized states (either
excitons or empty molecular orbitals) to say the pre-edge peaks
are really quasi-bound transitions, but below the real continuum.  
I'll try to avoid that debate.

I couldn't tell whether there was a large pre-edge peak followed
by the main absorption edge or a very large white line. If I
understand, it sounds like Feff is trying to put E0 at the
pre-edge peak... is that correct?

I'm sure that leaves unanswered questions, but I hope it helps 
show the general level of confusion about E0.

--Matt

On Tue, 8 Jun 2004, it was written:

> Dear Bruce and other mailing list users,
>  
> I have a very basic question concerning the choice of E0 in EXAFS data
> analysis. Recently I have been working on Ru solvated complex. I don?t
> go into too many details but I can say it?s a pretty large molecule in
> aqueous solution with 3 ligands containing 6 N atoms and 30 C atoms in
> D3 symmetry. Now, we have measured the L3 edge XANES and EXAFS of this
> molecule and I have recently tried to analyze it. Now, what bothers me a
> lot is the following observation: once I start processing the data
> (meaning background removal, pre-edge subtraction etc.) AUTOBK in both
> Ifeffit and Athena ?proposes? to place the edge energy E0 at the
> inflection point of the spectrum which lies at about 2838-2839 eV (which
> corresponds to atomic Ru L3 edge absorption energy). You might say it?s
> ok but I cannot accept this value. The inflection point lies on a huge
> pre-edge peak which is the 4d bound-to-bound transition (white line)
> which in principle I don?t want to include in my chi(k) because it?s
> still below the ionization energy as I understand it. I guess the
> continuum states should start higher in energy and actually from other
> spectroscopies one would suspect the E0 to be around 2847 eV (so about
> 7-8 eV higher in energy space). Now I set my E0 to this value and I
> started fitting the spectrum with FEFF calculation performed on
> crystalline structure of the same complex. What I end up with is always
> E0 correction for the first shell of neighbors (meaning N atoms) between
> -6 to -7.5 eV. I check in chi.dat and xmu.dat files that in my FEFF
> calculation the k=0 value was assigned to approx. 2839 eV so it?s the
> same atomic value again. However I know it shouldn?t be like that (apart
> from a simple fact that Ru in my compound is in its 2+ oxidation state
> which surely moves the E0 toward higher energies, doesn?t it?).  So, I
> tried to fool FEFF and make some tricks like shift my data in energy
> space or fit the white line and subtract it form the data but it always
> consistently converges to place the E0 value at the first inflection
> point meaning between 2839-2842 eV. I thought that when fitting the
> experimental data with FEFF scattering paths one can choose E0 and then
> refine its value by shifting the FEFF-calculated E0 by the amount which
> comes out of the fit. So if I have some value of E0 coming out of my fit
> then I just shift the initial k=0 value as estimated by FEFF using k? ->
> sqrt( k-E0(2m/h_bar^2) ) (in other words the negative E0 correction
> would always shift the origin in k-space towards larger k values which
> means lower values in energy space). Ok, so now my question is the
> following: is there an explicit way to set E0 value before letting FEFF
> to do its job or it will always use the tabulated atomic value? So, if I
> don?t know my E0 a priori how can I refine it for a complex or molecule
> which has different oxidation state than 0? I mean it?s a serious
> question in my opinion and I wish I could get some feedback from people
> who have been doing it for so many years.
> I hope I made my point clear enough to you. Let me know if you need any
> more feedback in order to answer my question. I?ll be glad to give more
> details.
> Best regards,
>  
> Wojciech
>  
> *********************************************************************
> Wojciech Gawelda
>  
> Laboratoire de Spectroscopie Ultrarapide (LSU)
> Institut des Sciences et Ing?nierie Chimiques (ISIC),
> Facult? des Sciences de Base (SB-BSP)
> Ecole Polytechnique F?d?rale de Lausanne (EPFL)
>  
> CH-1015 Lausanne-Dorigny, Switzerland
> Tel.:  +41 (21) 693 0452
> Fax.: +41 (21) 693 0422 
> E-mail:  <mailto:wojciech.gawelda at epfl.ch> wojciech.gawelda at epfl.ch
> WWW:  <http://lsu.epfl.ch> http://lsu.epfl.ch
> *********************************************************************
>  
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