Re: [Ifeffit] Phase shift

[Matt Newville]

Hi Julian,



On Tue, Apr 10, 2018 at 1:30 PM, Julian Ehwald  wrote:

> Dear all,
>
>
>
> I have a very general question about the phase shift. I tried to fit a
> rather noisy sample of Li2IrO3, where I get a first main peak around 1
> angstroem, according to FEFF calculation there should be something at 1.9
> and not any closer. This phase shift seems a bit too big to be true, is
> there something like an upper bound for a reasonable phase shift?
>
>
>
> Best, Julian
>

It's a little hard for me to tell what you mean.  Are you saying that Feff
predicts the peak of |chi(R)| to be at 1.9 Ang, or that the first shell
distance should be 1.9 Ang?

Phase-shifts for single-scattering peaks typically cause the peak of
|chi(R)| to be ~0.5 Ang below the near-neighbor distance. That can vary
some, but I would be surprised for it to be as big as 1 Ang.

It might be useful to post the data.

--Matt
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[Ifeffit] Phase shift

[Julian Ehwald]

Dear all,

 

I have a very general question about the phase shift. I tried to fit a
rather noisy sample of Li2IrO3, where I get a first main peak around 1
angstroem, according to FEFF calculation there should be something at 1.9
and not any closer. This phase shift seems a bit too big to be true, is
there something like an upper bound for a reasonable phase shift?

 

Best, Julian

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Re: [Ifeffit] Sulfur EXAFS k oscillations

[Jeffrey Catalano]

Actually, I have seen effects this like with the Spline Clamps turned up 
like you have them. What does this look like if you set Spline Clamps, 
both low and high, to none? Also, you should plot the full XAFS (in E, 
the whole thing, with the edge) and look at the background curve. My 
guess is it is oscillating and not doing a good job splining the data.


Jeff

On 4/10/2018 12:36 PM, Carlo Segre wrote:


Hi Beth:

I suggest that you not use kw=3 as that will make these background 
subtraction oscillations worse.


Another thing you can try is to cut down the high end of your spline 
range to 13, then 12.5 and see if that makes it behave a bit better.  
I find that it helps a lot when the data is a bit noisy or there is a 
slightly wavy background.  I also look at the FT to see how the 
background spline is doing.


Carlo

On Mon, 9 Apr 2018, Miller, Elizabeth Christine wrote:


Hi all,

I am analyzing some sulfur K-edge EXAFS data taken at 4-3 at SSRL. 
When I use a k-weight of k^3 to plot, my oscillations at high k reach 
over 100, which doesn't make sense to me. I have looked over the data 
with Riti, and it seems that while the XANES are rather self-absorbed 
due to high concentrations of sulfur-containing salt, the data can be 
normalized, and the EXAFS should contain some information. I've 
included screenshots in the attachment of plots of all of the 
k-weights as well as my parameters. The mixtures are solutions of 
organics (acetonitrile, 1,1,2,2-Tetrafluoroethyl 
2,2,3,3-tetrafluoropropyl ether) with 
bis(trifluoromethane)sulfonimide lithium salt. I am also not hitting 
any other edges, as sulfur is the highest Z element in the solution. 
Any thoughts would be greatly appreciated!


Thank you,
Beth



Elizabeth C. Miller, Ph.D.
Postdoctoral Scholar
Stanford Synchrotron Radiation Lightsource
SLAC National Accelerator Laboratory

E-mail: ecmil...@slac.stanford.edu






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Re: [Ifeffit] Sulfur EXAFS k oscillations

[Carlo Segre]

Hi Beth:

I suggest that you not use kw=3 as that will make these background 
subtraction oscillations worse.


Another thing you can try is to cut down the high end of your spline range 
to 13, then 12.5 and see if that makes it behave a bit better.  I find 
that it helps a lot when the data is a bit noisy or there is a slightly 
wavy background.  I also look at the FT to see how the background spline 
is doing.


Carlo

On Mon, 9 Apr 2018, Miller, Elizabeth Christine wrote:


Hi all,

I am analyzing some sulfur K-edge EXAFS data taken at 4-3 at SSRL. When I use a 
k-weight of k^3 to plot, my oscillations at high k reach over 100, which 
doesn't make sense to me. I have looked over the data with Riti, and it seems 
that while the XANES are rather self-absorbed due to high concentrations of 
sulfur-containing salt, the data can be normalized, and the EXAFS should 
contain some information. I've included screenshots in the attachment of plots 
of all of the k-weights as well as my parameters. The mixtures are solutions of 
organics (acetonitrile, 1,1,2,2-Tetrafluoroethyl 2,2,3,3-tetrafluoropropyl 
ether) with bis(trifluoromethane)sulfonimide lithium salt. I am also not 
hitting any other edges, as sulfur is the highest Z element in the solution. 
Any thoughts would be greatly appreciated!

Thank you,
Beth



Elizabeth C. Miller, Ph.D.
Postdoctoral Scholar
Stanford Synchrotron Radiation Lightsource
SLAC National Accelerator Laboratory

E-mail: ecmil...@slac.stanford.edu




--
Carlo U. Segre -- Duchossois Leadership Professor of Physics
Interim Chair, Department of Chemistry
Director, Center for Synchrotron Radiation Research and Instrumentation
Illinois Institute of Technology
Voice: 312.567.3498Fax: 312.567.3494
se...@iit.edu   http://phys.iit.edu/~segre   se...@debian.org
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Re: [Ifeffit] [Ifeffit} Sulfur EXAFS k oscillations

[fred.mosselm...@diamond.ac.uk]

Dear Beth,
Apart from the fact it looks like there is a trace Cl impurity in one of the 
samples (sharp line at ca. 9.5 k),
it is hard to tell how a spline is going wrong without seeing the spline, thus 
posting the Athena file might get a better response.
It may well be there is not much data beyond 10 k for soft scatterers at the S  
K-edge.

Best wishes
Fred


J.F.W. Mosselmans
Principal Beamline Scientist I18
Diamond Light Source Ltd
Diamond House
Harwell Campus
Didcot
OX11 0DE

P + 44 1235 778568
M + 44 7785510211
E fred.mosselm...@diamond.ac.uk
orcid.org/-0001-6473-2743






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