[Ifeffit] E0 issues

Scott Calvin dr.scott.calvin at gmail.com
Mon Jun 27 19:27:07 CDT 2011

Hi again,
> Thanks Scott,
> I think it is a mistake done by many beginners. To be confident that  
> I understood you well: so basically the reproducibility of E0  
> between samples while fitting to FEFF files mainly depends on the  
> quality of my calibration.

For reproducibility, yes.

> Actually, if there is no clear feature in my edges and they are  
> different in ‘slopes’, I must have either a calibration foil, or a  
> known standard, measured at the same time; otherwise there is no  
> good way to get comparable data?

Some beamlines are more stable in energy than others. Sometimes it  
suffices to measure a standard occasionally between measuring data. If  
the calibration does not drift, or drifts in a predictable way, then  
you're OK. But if the calibration jumps around by an eV or two, as is  
not uncommon, then your data inherits that uncertainty unless you  
measure a reference material at the same time.

> Is that right, or the fitting, considering all spectra’s components  
> will finally lead to a good fit, even with relatively poor   
> calibration (I will definitely be more careful in the future; but  
> asking for a set of samples that were mistakenly measured for me  
> without a standard).

This is a different question. If you are fitting to FEFF, then it  
doesn't matter if the E0's were defined consistently for each sample,  
or if the calibration drifted. You will most likely float E0 as a free  
parameter anyway, so the fitting process will adjust for the  
differences. You do lose the ability to compare the E0's of your  
different samples, and you lose the ability to constrain them to be  
the same, but the rest of the fitting process is fine.

--Scott Calvin
Sarah Lawrence College

P.S. Also note that if ifeffit returns an E0 shift of more than about  
10 eV, that's a warning sign. Check if that would correspond to an E0  
still on or near the rising portion of the edge (a bit past the white  
line is still OK). If it's not, then the fit is not a good one. If it  
is, then it's best to choose a new E0 in Athena (or SixPack) that is  
closer to to where ifeffit wants it; FEFF loses accuracy when the E0  
has to be shifted by that much.

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