[Ifeffit] FEFF and XANES

John J. Rehr jjr at leonardo.phys.washington.edu
Mon Dec 11 16:45:14 CST 2006


Hi Jakob,

As you have noted the fine structure in the projected DOS and the XAS
is the same, so  the relation between them is:

   XAS =  gamma(E) LDOS, where   gamma(E) = mu_0(E)/rho_0(E),

where gamma(E) is a smoothly varying atomic-like quantity which
is essentially a ratio of embedded atomic matrix elements.  In systems with
a white line $mu_0$ and hence, gamma(E) will likely be peaked at threshold.
As a result the XAS is smoothly related to the LDOS with appropriate
selection rules.  This discussion assumes core-hole effects are small,
and final state broadening is applied both the the LDOS and XAS.

   J. Rehr

On Mon, 11 Dec 2006, Frommer  Jakob wrote:

>
> Dear all,
> I have a FEFF related question (I know that this is an IFEFFIT list, but I guess the subjects are closely related). I try to model some of my Cr XANES data with FEFF (following the procedure outlined in an excellent publication in Chem Phys 300 (2004) 13-22 – the authors have shown that it is possible to explain the Cr XANES spectra at least qualitatively by the means of FEFF calculations). I have however still some trouble to understand the difference (or the relation) between the XANES results given in the xmu.dat file and the l-projected density of states results (ldosXX.dat). Due to rho(E)=rho_0(E)[1+chi(E)] I would expect them to conincide (if the rho is calculated for the correct l – i.e. if the matrix element is non-zero). However I observed that the ldos and the xmu do not always coincide – is this possible? Furthermore at the white line position (from what I know, the white line is commonly interpreted as being due to the transition into unoccupied but bound states)!
> the xmu shows strong intensity whereas the ldos do not (maybe one comment: I assume that the e-scales in the ldos-file and in the xmu-file are the same).
> I am using FEFF8.2 with the SCF, DOS, XANES and FMS card.
> I would be obliged for your help.
> Thanks
> Jakob
>
> P.s.: for hydroxide structures FEFF is always returning “bad counts” if I include the protons into the FEFF calculation – however this might be also due to bad input structures.
>
>
> ___________________________________________________
>
> Jakob Frommer
> Soil Chemistry Group
> Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich
> Universitätstrasse 16, CHN F19
> CH - 8092 Zürich
> Tel: +41 44 632 87 58
> Fax: +41 44 633 11 18
> Email: jakob.frommer at env.ethz.ch
> http://www.ito.ethz.ch/SoilChem/
>
>
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