[Ifeffit] the shortest possible K-range ,

[Matt Newville]

Hi Mahmoud,

On Tue, Apr 18, 2017 at 5:11 AM, Al Humaidi, Mahmoud <
mahmoud.alhumaidi at student.uni-siegen.de> wrote:

> Hi ,
>
> I want to ask please what is the shortest k-range one can use for "Forward
> Fourier transform"
>
> in my projects I used:
>
> Spline range k 0.0 - 9.0
> k range in Forward Fourier transform 1.0 - 9.0 A^-1
>

That should be OK, though it will limit the resolution and number of
parameters you can use to model the data.

Starting at k=1 Ang^-1 is often considered to be a bit too low for EXAFS
analysis, as this is generally still in the XANES region where multiple
scattering contributions dominate and where the background subtraction is
more uncertain.   That is, starting a k=2 or 3 Ang^-1 might be a better
choice.



>
> I am trying to reduce the contribution of noise in FT ..
>
>
Well, limiting the k-range does not always reduce the noise in the part of
the signal you care about (R= 1 to 6 Ang).  It can, but just because
k^2*chi(k) looks noisier at high k does not necessarily mean the data is
only noisy in that range.  Mostly the noisiness at high-k implies that the
signal-to-noise ratio is reduced, but that's at least partially because the
signal goes down and may not mean the noise is going up.  OTOH, the
measurements sometimes do get noisier at higher energy.....


> the attached project shows nice fit ,,,but high phase shift toward lower R
> value ,,
> any recommendation please I will appreciate ..
> and thank you in advance ,
>
>
I don't understand why your "9%" merge data gets weird when making the
spline go past 10 Ang^-1, but the "3%" ad "5%" merge data looks sort of OK
to me going out to 11 or 12 Ang^-1.   I am not sure the signal is real....
If these are merged data, I'd suggest looking at the scan to scan
variations to decide if the data is real.

Hope that helps,

--Matt




> best regards
> Mahmoud AL Humaidi
> ------------------------------------------------------------
> ---------------------
> *Mahmoud Al Humaidi *
>
> Department of Physics, University of Siegen
> Emmy-Noether-Campus
> Walter-Flex-Str. 3, 57072 Siegen
> Room : B-015
> Tel :  +49 (0)  271 740 3727 <+49%20271%207403727>
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