Re: [Ifeffit] Multi-electron peak in Yb

[Matt Newville]

Hi Matthew,

Attached is a file of Yb L3 edge on something that was labeled "Yb AC" and
measured in fluorescence many years ago, almost certainly in solution.  I
suspect that "AC" means "acetate".

This is a plain ASCII file (not XDI!).   The data quality is not great.
Does this have the feature you're seeing?

--Matt


Yb_Ac_starndard.dat
Description: Binary data
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Re: [Ifeffit] Multi-electron peak in Yb

[Matthew Marcus]

The samples this guy is interested in are synthetic.  No Cu visible.  Cu XANES 
tends not to have huge white lines, which is what would be
required for the contamination to show as a peak with no detectable edge jump.  
Also, the peak is nowhere near the Cu K-edge.  For similar
reasons, it's not Fe either.
mam

On 8/4/2016 4:20 PM, Matt Newville wrote:

Hi Matthew,


On Thu, Aug 4, 2016 at 2:24 PM, Matthew Marcus > wrote:

Does anyone know of data on multi-electron peaks at the Yb L3 edge?  One of 
the users here was just running Yb and finds on every spectrum a bump at the 
same
energy.  It's not a Bragg glitch from the sample or an I0 glitch or any 
other obvious artifact.  It reminds me of the two little bumps at Ce.  If 
someone has
analyzed and parameterized this peak as a fraction of edge jump, then it 
would be possible to subtract it off.  The attached papers show exmples for 
almost every
lanthanide *execpt* Yb.


I have not done much work on Yb, and all the measurements I can find are on the 
L2 edge, not the L3 edge.


That's because the L3 edge is at 8944, 30 to 40 eV below the Cu K edge.   In my 
experience, it's really hard to get rid of Cu and Fe as trace contaminants, 
especially in natural samples.Is it possible that what you're seeing is not 
multi-electron excitation by Cu contamination?




For those doing Ce, I've written a program which does the correction, 
essentially as presented in the attached Gomilsek paper.


That sounds useful.

--Matt



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Re: [Ifeffit] Multi-electron peak in Yb

[Matt Newville]

Hi Matthew,


On Thu, Aug 4, 2016 at 2:24 PM, Matthew Marcus  wrote:

> Does anyone know of data on multi-electron peaks at the Yb L3 edge?  One
> of the users here was just running Yb and finds on every spectrum a bump at
> the same
> energy.  It's not a Bragg glitch from the sample or an I0 glitch or any
> other obvious artifact.  It reminds me of the two little bumps at Ce.  If
> someone has
> analyzed and parameterized this peak as a fraction of edge jump, then it
> would be possible to subtract it off.  The attached papers show exmples for
> almost every
> lanthanide *execpt* Yb.
>
>
I have not done much work on Yb, and all the measurements I can find are on
the L2 edge, not the L3 edge.   That's because the L3 edge is at 8944, 30
to 40 eV below the Cu K edge.   In my experience, it's really hard to get
rid of Cu and Fe as trace contaminants, especially in natural samples.
Is it possible that what you're seeing is not multi-electron excitation by
Cu contamination?



> For those doing Ce, I've written a program which does the correction,
> essentially as presented in the attached Gomilsek paper.


That sounds useful.

--Matt
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Re: [Ifeffit] Questions about fitting of EXAFS

[Fuxiang Zhang]

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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Re: [Ifeffit] Questions about fitting of EXAFS

[Scott Calvin]

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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Re: [Ifeffit] Questions about fitting of EXAFS

[Bruce Ravel]

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

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-physics-nanoscience-and-mesoscopic-physics/introduction-xafs-practical-guide-x-ray-absorption-fine-structure-spectroscopy?format=HB


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

B

--
 Bruce Ravel   bra...@bnl.gov

 National Institute of Standards and Technology
 Synchrotron Science Group at NSLS-II
 Building 743, Room 114
 Upton NY, 11973

 Homepage:http://bruceravel.github.io/home/
 Software:https://github.com/bruceravel
 Demeter: http://bruceravel.github.io/demeter/
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