[Ifeffit] reply to: RMULTIPLIER-Card (edge shifts/bond lengths)

grant bunker bunker at iit.edu
Sat Jul 5 16:07:09 CDT 2008


RE the effect of bond length on edges: this physical effect is normal  
and has been known for many decades. Although the XANES is a composite  
of mu_zero, single scattering, and multiple scattering, there is an  
underlying simple effect of bond length.  Broad features on K-edges  
(which are dominated by mu_zero and single scattering) shift  
approximately as delta E ~ k^2 ~ 1/r^2, where r is the average nearest  
neighbor bond length, simply from the phase advance in the single  
scattering component. The phases of the higher frequency oscillations  
from multiple scattering advance more rapidly when the bond length is  
increased.  L edges are a bit different because there are often bound  
states near the fermi level that have to be accounted for.

I studied this in my thesis quite a while ago (gb thesis 1984) in the  
pre-FEFF era.  Theoretically, by freezing the charge densities of the  
atoms, changing the bond lengths (and other geometrical parameters),  
and then calculating the XANES in both single scattering and multiple  
scattering approximations shows that it is primarily a geometrical  
effect, for the reason I outlined above. It also is in accord with  
experiment as I showed with Mn oxides.

A basic introduction can be found here: http://gbxafs.iit.edu/training/XANES_intro.pdf

grant bunker

On Jun 23, 2008, at 9:34 AM, ifeffit-request at millenia.cars.aps.anl.gov  
wrote:

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>   1. RMULTIPLIER-Card (s440697 at stud.uni-goettingen.de)
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> Message: 1
> Date: Mon, 23 Jun 2008 16:33:50 +0200 (CEST)
> From: s440697 at stud.uni-goettingen.de
> Subject: [Ifeffit] RMULTIPLIER-Card
> To: ifeffit at millenia.cars.aps.anl.gov
> Message-ID:
> 	<53054.134.76.41.165.1214231630.squirrel at webmail.stud.uni-goettingen.de 
> >
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> Content-Type: text/plain; charset="iso-8859-1"
>
> Hi,
> I have a question concerning the RMULTIPLIER-Card in XANES- 
> Calculations. I
> made some calculations with different rmult values (1.00, 1.01, 1.05,
> 1.10) and found a shift of the absorption-edge or rather the slope  
> of the
> peak. I attached the diagram of this calculation (Sulfur-K-edge of
> gypsum).
>
> My question is, what could be a physical reason for this behaviour?  
> With
> this Parameter one change the interatomic disctances. Hence the
> interference phenomena and the curve progression should be change,  
> too. Is
> this right?
>
>
>
> Thanks for your help.
>
> Have a nice day,
> Eckhard
>
> PS: the same behaviour occurs in a inverse sense for rmult < 1
>
> ---------------------------------
> Eckhard Bosman
> e.bosman at stud.uni-goettingen.de
> +49 (0)551-39-14441
> Raum: E0.104
> Institut f?r R?ntgenphysik
> Friedrich-Hund-Platz 1
> 37077 G?ttingen
> Germany
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