[Ifeffit] Noisy spectrum and AUTOBK

[gabriel rosenblum]

Hi all

I have a question regarding noisy EXAFS, since I
analyze biological zinc samples. I have seen Bruce’s
data and it is quiet as a sleeping snail all the way
till chi(k)=16, however I have a completely different
situation since I encounter noise at high chi(k) and I
need an advice from whoever can supply it.
As I use AUTOBK with emax set to maximum I observe
chi(k) which is quiet at 0-9, and at 9-12 I obsrerve
clearly the oscillations but with the addition of
small spikes resulting from noise. At chi (R ) I see
the first shell peak at 1.5A with amplitude of 1.
Adjacent to this peak I observe a small peak at 1A
with 0.5 amplitude. As I change Rbkg and Knin I cannot
get rid of the small peak; the left region of the
small peak is situated between 0.5 to 1A at chi(R ),
and my attempts to keep this region “clean” failed.
However, if I lower emax to 400, corresponding to
approx. 10 at chi(K), and fourier-transform the region
between 2 to 9 (the quiet region), the small peak
disappears and the region at Chi (R ) between 0 to 1A
is “clean”, but since I utilize less information the
peaks at chi (R ) have lower resolution. 
My questions are:
1.	What is the source of the small peak ?
2.	Is it a real contribution from scattering ligands ?
3.	How should I treat the small peak.
4.	Should I get rid of it by all means ? if so, how ?

My confusion arises from the fact that AUTOBK should
get rid of atomic exafs at chi(k), consequently, the
contributions at chi (R ) between 0 to 1A should get
lower. In my case it seems that this contribution is
lower, but it still exist with half the amplitude of
the first shell peak.
Any help would be appreciated.
Thanks.
Gabi.



	
		
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