[Ifeffit] a small question

[Matt Newville]

Shaofeng,


On Wed, Nov 4, 2015 at 6:29 AM, Shaofeng Wang <wangshaofeng at iae.ac.cn>
wrote:

> Dear all experts,
>
> I am processing two As K edge EXAFS data  shown as following figure. The
> first shell is As-O. The difference of the As-O peak btween sample As1 and
> As2 is around 0.03A. Does this represent that the average As-O bond length
> of sample As1 should be around 0.03 A longer than that of sample As2? Or
> this depends on fitting results ?
>


If some assumptions hold, it probably is a good indication that the bond
length is probably a bit longer.  The main assumptions would be a) that you
know the As is surrounded by only oxygen, and  b) that the data k-range,
Fourier transform parameters, and value for E0 are exactly the same for the
two data sets.

Even with those assurances, the position of maximum of |chi(R)| is
difficult to determine with high accuracy.  In fact, for almost all work
with Artemis, the grid spacing for the chi(R) data is ~0.0307 Ang
(pi/10.24), which is suspiciously close to your observed difference.

As an alternative, to looking at the position of the maximum of |chi(R)|,
it might be more precise to use the position where the imaginary part of
chi(R) crosses zero.  This should be the same position, but allows you to
use linear interpolation to find the location of that zero.

A full fitting of the As-O shell has the ability to be more accurate and
use all data, not just one position of chi(R).   Still, such differences in
where Im[chi(R)]=0 can be a useful guide for further analysis.

Cheers,

--Matt




> Any help is very appreciated.
>
>
> [image: image]
>
> Cheers,
>
>
> Shaofeng
> --------------------------------------
> Shaofeng Wang, Ph.D of Geochemistry
> Environmental Molecular Science Group
> Institute of Applied Ecology, Chinese Academy of Sciences
> Shenyang, 110016, China
> wangshaofeng at iae.ac.cn
> www.iae.cas.cn
>
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