[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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