CORRECTIONS imaginary shift enquiry

[David Eustace <daveus@chem.gla.ac.uk>]

Dear Feffusers,

Seasons greetings to all!

I'm writing hoping for a bit more information on the CORRECTIONS (and 
EXCHANGE) cards, specifically regarding the mechanism used to broaden XANES 
spectra.

I've been simulating Oxygen K-edge XANES for various spinel compounds, and 
find that some amount of imaginary broadening is needed to obtain a better 
fit to experimental data. To do this I run the CORRECTIONS card, with the 
first (real) shift set to 0.0, and the imaginary shift set, usually, to 
some value between 0.5eV and 1.2eV. Experimental broadening should not be 
beyond 0.5eV.

I find that the increase in peak width does not reflect my perception of 
what 0.5eV broadening means (this is my error, I know). Previously sharp 
peaks in the XANES become much broader, and peaks merge seemingly beyond 
the 0.5eV parameter. I know this is an amateurish description of the 
fitting process, but I'm just trying to get my point across simply. I would 
have expected a Gaussian Full Width Half Maximum type broadening process, 
but my data does not reflect this.

If some kind soul could provide me with a description of how 
(mathematically) the imaginary shift is broadening the spectra, it would 
really help my peak fitting process a lot.

 Thanks, and Merry Christmas,

David Eustace
University of Glasgow