[Ifeffit] E0 for bulk crystals and nanoparticles

[Frenkel, Anatoly]

Leandro - depending on the details of your system and treatment, electronic structure of the nanoparticle may or may not change at elevated temperatures. It may change as a result of hydrogen chemisorption, charge exchange with support, ligand desorption, oxidation, etc. As a result, you may observe changes in XANES that may be due to the change in core hole screening or Fermi energy change or both (depending on the degree of "metallicity" of the nanoparticle).
 
There are many references in literature demonsrating each effect that I mentioned.
 
Thus, keeping E0 the same at all temperatures may or may not be wise, depending on the magnitude of the edge shift that can, in principle, be observed at different temperatures. If that happens, you may want to compare the resuls obtained by keeping E0 the same vs those when E0 was allowed to vary independently for each scan.
 
In short, there is no "one fits all" answer.
 
Anatoly

________________________________

From: ifeffit-bounces at millenia.cars.aps.anl.gov on behalf of Leandro Araujo
Sent: Sun 2/25/2007 9:44 PM
To: Bruce Ravel; XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] E0 for bulk crystals and nanoparticles


Hi Bruce,

thank you very much for taking the time to write your helpful reply.

As you mentioned,


	This really gets at the heart of why exafs analysis is difficult.  You
	are certainly correct that the deltaE0 is highly correlated with delR
	and C3.  That correlation is hard to avoid in a non-linear analysis
	as all three terms effect the phase of the calculated chi(k). 


That's exactly why I decided to post the question on the list asking for the advice of more experienced people who can certainly evaluate the question much better than myself.


Also, I've always been trying to follow your "reasonable and defensible applied to all the parameters, not just e0" motto; another strong motivation for posting this question on the list. I don't know if it is just me, but in my short time working with EXAFS I got the strong impression that it is very easy to fall into the temptation of sophistry to get to the "expected" results from a fit. In this respect, I consider the mailing list an invaluable resource to help overcoming such a temptation. 

But coming back to the original question, would you say that analyzing a temperature-dependent set of the very same NC-containing sample (eight measurements between 10K and 300K) to get an E0 value would be a sound approach? Or should we expect E0 to vary with the temperature as well? 

Cheers,
Leandro


-------------- next part --------------
A non-text attachment was scrubbed...
Name: winmail.dat
Type: application/ms-tnef
Size: 5593 bytes
Desc: not available
URL: <http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20070225/92ecd0cc/attachment.bin>