[Ifeffit] EXAFS simulations - peak too high

Scott Calvin scalvin at sarahlawrence.edu
Mon Feb 3 12:58:48 CST 2014

Hi Lisa,

Without thinking too much about your particular structure (I wouldn't be surprised if someone on the list chimes in with that kind of info by the time I finish composing this!), I am very confident it is not a fault in the FEFF code.

Ad you probably know, chi(R) is NOT a radial distribution function, although they are related. For one thing, multiple-scattering can play a role, and it may be that in one of the sites multiple scattering may be enhancing the peak--perhaps there are more sulfur atoms collinear with the gold-gold paths, for instance. In addition, chi(R) is complex valued, meaning that different paths can reinforce or cancel in a non-linear way if you just look at the magnitude. In other words, the chi(R) magnitude of the combination of Au1 - Au2 and Au1 - Au3 might be larger than the sum of the individual magnitudes of the individual magnitudes or smaller.

About a decade ago, I analyzed the structure of elemental manganese using EXAFS. Elemental manganese has one of the nuttier crystal structures you'll find in a pure element, with four crystallographically distinct absorbing sites. Because of interference between scattering paths, the "importance" of each site to the fit varied by up to 400% from what would be expected due to abundance.

I never published that result, but I do have the pdf of my presentation to the APS on it, so I'm attaching that.

I'd guess your effect is either something like that or an effect of multiple-scattering.

--Scott Calvin
Sarah Lawrence College

On Feb 3, 2014, at 1:27 PM, Lisa Bovenkamp <lbovenkamp at lsu.edu> wrote:

> Hello.
> I am running some EXAFS simulations Au L3 on Au clusters with SR coating.
> In the outer shell Au sits between 2 S atoms. 4 Au atoms are in the neighborhood
> up to 3.5 Angström. 
> Expecting two peaks in the chi(R) - one for Au-S and one for Au-Au (here 
> it can be several little ones) I find that for 1 out of 5 sites the Au-Au
> peak is way too high. 
> Is something in the FEFF code weighing an Au-Au path for this site
> too much? Does anyone have some ideas on that?
> Thanks,
> Lisa

-------------- next part --------------
A non-text attachment was scrubbed...
Name: MnAPSpresent.pdf
Type: application/pdf
Size: 666800 bytes
Desc: MnAPSpresent.pdf
URL: <http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20140203/eda34181/attachment.pdf>

More information about the Ifeffit mailing list