[Ifeffit] XANES pre-edge vs. edge features

[Bruce Ravel]

On 01/25/2016 09:13 AM, Aditya Shivprasad wrote:
>
> I was looking at the XANES standard for Fe foil from Hephaestus and I
> noticed that there was a small, curved feature at the edge (7112 eV),
> another inflection point at 7116.4 eV, and then the edge step at around
> 7131 eV. My question is: why is the feature at 7112 eV considered as the
> edge and not as a pre-edge feature? Are they due to fundamentally
> different phenomena? I would like to understand where this type of
> feature comes from so as to be consistent in the current paper that I am
> writing. I have attached the standard, just in case.

Hi Aditya,

That's a great question!

In Athena, you are looking to identify the "threshold" or the "zero of 
wavenumber" as the value for E0.  In a metal, this is probably not hard 
to identify.  In a compound, which might have localized, unoccupied 
states, the question is a bit harder.

As a practical matter, you want to process your data consistently and in 
a way that yields defensibly normalized data so that you can use XANES 
methods (e.g. LCF or PCA) and EXAFS analysis in a defensible way.  As a 
practical matter -- almost any point on the main rising edge tends to be 
adequate for either XANES or EXAFS analysis.

I was going to go on and talk about how to think about the Fermi energy, 
but I was going to say basically the same thing that Matt said in his 
email that arrived while I was typing.

Matt's comment about the "XANES industry" is quite right and I encourage 
you to read up on that.  But as a practical matter, these choices should 
be consistent such the normalization is done in a consistent manner.

What do I mean by "consistent"?  Well, I believe you said you are doing 
some kind of LCF analysis of your data.  Well, if you were to make a 
synthetic sample with known quantities of Iron standards, then your 
methodology for the analysis of the standards and the synthetic sample 
should yield the same ratios that were used to make the sample.

HTH,
B


-- 
  Bruce Ravel  ------------------------------------ bravel at bnl.gov

  National Institute of Standards and Technology
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