[Ifeffit] E0 issues

[Hana]

Scott Calvin <dr.scott.calvin at ...> writes:

> 
> Hi Hana,
> > On Jun 27, 2011, at 2:04 PM, Hana wrote:
> > > I guess my question is obvious to most of you, but after some  
> > practice and  reading, the following is still unclear to me  
> > (hopefully some other beginners  will benefit): Normally I have  
> > calibrated my spectra using a calibration foil –  so energy shift  
> > was done according to that companion standard. Doing a simple   
> > linear combination fit, I have set all spectra to the same E0 value,  
> > somewhere on the edge, above the first derivative maxima (was it a  
> > good practice?).
> > That is a good practice, in my opinion. If you're going to do a linear  
> combination fit, it doesn't really matter how you choose E0, as long as:
> > 1) The spectra are all aligned on the same energy scale
> > 2) The choice of E0 is in some way consistent
> > By using a calibration foil, you have assured #1, and then by simply  
> making E0 the same for all spectra, you have assured #2.
> > > Recently, I found myself actually confused about E0; since my  
> > samples inherently have a phase difference (which is the base to my  
> > ability to differentiate them), how a certain reference point on the  
> > spectrum can be determined?
> > "Reference point" is indeed the correct term. As such, it is somewhat  
> arbitrary, and just needs to be consistent for all spectra being  
> compared.
> 
> > What can be done when I do not have the calibration foil (especially  
> > for these heavy elements that do not have specific sharp feature)?
> > Consistency is the only requirement. There are many ways to align  
> consistently. If you've got really noisy reference data for some  
> reason, you could even fit some kind of function to the edge and use  
> that, but for reference data that's not usually necessary.
> 
> > And further, now that I am starting to work on the structural model;  
> > how actually IFEFFIT determined the energy shift when there is no  
> > specific reference point?
> > If you float E0 when fitting to feff files, the reported value is a  
> shift relative to wherever you initially picked it. Thus if you change  
> the initial choice of E0 by 2 eV upward, the shift ifeffit reports  
> should be 2 eV less.
> 
> Hope that helps...
> 
> --Scott Calvin
> Sarah Lawrence College

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. 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? 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).
Thanks!
>