This point may have come up before in this thread or in one of the numerous links, but I'd like to note that the FEFF version used by default in Artemis usually requires large E0 for heavy elements like Pt. Often, the E0 that results in small values of enot is at or above the white line. An easy way to demonstrate this is to grab a spectrum for Pt or Au foil and fit the first shell. mam On 8/4/2016 8:07 AM, Bruce Ravel wrote:

On 08/03/2016 05:36 PM, Fuxiang Zhang wrote:

...

Hi, Everyone, I have several detailed questions about EXAFS processing and fitting 1. Are there any physical meaning for DeltaE0 in the paths, why it should be less than 10eV

This paper is a good explanation of why Delta E0 should be reasonably sized:

https://doi.org/10.1107/S0909049598002970

...

2. The cutting range of Kmax (FT transform parameters) has great effect on FTs of EXAFS, how do I know to use the best value of Kmax;

If you have measured data with signal well above the level of noise, why would you choose to use less data?

Similarly, if, at some point in your data, the signal becomes dominated by noise -- either statistical or systematic -- why would you include it in the analysis?

...

3. How to make refinement equation for multi-scattering paths? Such as in NiFe alloys (simple fcc structure), the path [Ni-FM-3m] Fe1.1 Ni1.4 Fe1.1., I use 0.67*Efe+0.33*Eni for DeltaE0, and 0.67*ssFe1+0.33*ssNi4 for sigma2, is that right? Many thanks

Have you been reading the messages posted on this mailing list? If so, you have certainly noticed that I often answer questions like this one in terms of whether or not a fitting result is "defensible". For example:

http://www.mail-archive.com/ifeffit%40millenia.cars.aps.anl.gov/msg05551.htm... http://www.mail-archive.com/ifeffit%40millenia.cars.aps.anl.gov/msg04067.htm... http://www.mail-archive.com/ifeffit%40millenia.cars.aps.anl.gov/msg00396.htm...

and many others.

Make use of the resources at

http://bruceravel.github.io/XAS-Education/ http://bruceravel.github.io/demeter/ http://xafs.org/Tutorials

and elsewhere.

Check out Scott's book,

https://www.crcpress.com/XAFS-for-Everyone/Calvin/p/book/9781439878637

or Grant's book

http://www.cambridge.org/us/academic/subjects/physics/condensed-matter-physi...

The library at your university may have either or both of those.

B

Thank you all for the answers. The Kmax effect on the shape of magnitude of FTs of one of my samples may come from the low signal/noise ratio. I still not sure if there's any physical meaning of DeltaE0, Does it relate to the shift of absorption edge? Fuxiang On Thu, Aug 4, 2016 at 1:35 PM, Scott Calvin wrote:

I’ll add to Bruce’s comment that if the choice of kmax has a great effect on the parameters found by the fit, that’s an indication of a problem! Good fits should be stable to modest changes in kmax (e.g., an inverse angstrom or two one way or another). The statistics may suggest one kmax or another is somewhat better, but the fitted parameters should not be drifting outside of the ranges defined by their uncertainties. If they are, you have an unstable fit. In that case, there are several possibilities: perhaps you are including an artifact in your k-range (e.g. the beginning of another edge) or a lot of data dominated by noise. Or perhaps your model is having trouble distinguishing between two fitting minima, and is flipping back and forth between them (this is more likely if your fitting model is fairly complicated, with many free parameters). In any case, if you’re fit is highly sensitive to kmax you should investigate to try to determine why.

—Scott Calvin Sarah Lawrence College

...

On Aug 4, 2016, at 11:07 AM, Bruce Ravel wrote:

...

2. The cutting range of Kmax (FT transform parameters) has great effect on FTs of EXAFS, how do I know to use the best value of Kmax;

If you have measured data with signal well above the level of noise, why would you choose to use less data?

Similarly, if, at some point in your data, the signal becomes dominated by noise -- either statistical or systematic -- why would you include it in the analysis?

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