[Ifeffit] E0 correction in EXAFS fitting

[Wojciech Gawelda]

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 Ingénierie Chimiques (ISIC),
Faculté des Sciences de Base (SB-BSP)
Ecole Polytechnique Fédérale 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
*********************************************************************
 
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20040608/4e07fdf3/attachment.html>