[Ifeffit] Inequivalent sites and multiple shell fits

[Paul Fons]

Thanks for the message.  I guess what I wanted to say was that from 
what I understand due to lifetime effects the "effective" radius about 
the absorbing site for which the outgoing scattered wave "exists" is on 
the order of 2 nn (I have big lattice constant material).  It would 
seem to me then that the inequivalent sites being physically separated 
by more than this distance (we are talking about crude approximations 
here) would not interfere.    In reality, with bond lengths on the 
order of 3 Angstroms and 2nn along the lines of 4.5 Angstroms, my case 
is somewhere in between the coherent interaction and the dilute dopant 
-- e.g. every site is by itself -- extreme.  Certainly, in the latter 
case it would be wrong to add the signals together coherently!  What is 
the consensus for this sort of thing?  The different E0's is a good 
point too.

						Paul



On 2005/01/14, at 16:16, Scott Calvin wrote:

> Hi Paul,
>
> Actually, the inequivalent sites do interfere coherently. EXAFS chi's 
> aren't like quantum mechanical wave functions, for which only relative 
> phase matters. The phase gives information about E0, and that's a 
> physically meaningful quantity. In other words, if one of the sites is 
> producing a peak at 7200 eV and another is producing a trough, they 
> will tend to cancel.
>
> So yes, Artemis and Ifeffit do the right thing, but the right thing is 
> to add everything up coherently regardless of which feff calculation 
> it comes from.
>
> A few other notes on this kind of fit: you must, of course, weight the 
> amplitudes of each feff  calculation by the relative number of sites 
> of that type. Also, it is quite possible that you'll need different 
> e0's for the different sites.
>
> --Scott Calvin
> Sarah Lawrence College
>
>
>
>   A simple question.  I am have been doing multiple edge fits of 
> different materials for a while in artemis (diana).  Now, I would like 
> to analyze a material with inequivalent sites for one of the edges.  I 
> understand how to read in the data and have read in the different feff 
> calculations (for each inequivalent site), but what I just want to 
> confirm that artemis/diana/ifeffit will do the "right" thing, namely 
> coherently add the contributions *within* each feff calculation and 
> add the intensities of each feff calculation -- e.g. I don't think 
> there is any interference between the XAFS signal on different 
> crystallographic sites -- or even if there is it is ignorable at least 
> for first neighbor interactions.  If there is anyone out there with 
> experience in doing this (or is the right thing done automatically?), 
> drop me (or the list) a line.  Thanks a lot!
>
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Dr. Paul Fons
Senior Researcher
National Institute for Advanced Industrial Science & Technology
METI
Center for Applied Near-Field Optics Research (CANFOR)
AIST Central 4, Higashi 1-1-1
Tsukuba, Ibaraki JAPAN 305-8568

tel. +81-298-61-5636
fax. +81-298-61-2939

email: paul-fons at aist.go.jp

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