[Ifeffit] E0 issues

[Scott Calvin]

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