Hi Gabi, I need some more information to address your concerns. See below:

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.

What is emax? Do you mean that you set Ezero to the maximum of the first derivative of the xmu data? What does the background function look like? Does it have a large spike right at the beginning? If so adjusting Ezero can help reduce that. Usually kmin should be small like zero. The exception being if you have a large white line (peak at the top of the absorption step). Then you want to increase kmin to help the bkg go over the white line

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 ),

If you increase Rbkg to 2.5 you must be able to get rid of the low r peak. Although I don't recommend that you do that, for real. Try it, just for fun and see. Often the low r peaks really don't matter. To get rid of them, the most robust method is to fit the first shell peak with a theory then pull the fit back into Athena and remove the background with the fit as a "standard". --see below for a recipe.

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 ?

I don't understand the emax variable. But, Since the low r peak disappears as you move the k-range up a bit then I believe the low r peak is an incorrect bkg removal that can be fixed by including a standard in Athena. Here is a recipe to get a nice background function. I'm sure Bruce and Matt have this written down somewhere too. 1) Fit the first shell in Artemis. Using a limited k-range where you are really confident that the oscillations are not distorted by the bkg. k=[2:9] determine a value for deltaE, deltaR and N or SO2. DetlaR, N or SO2, should be reasonable. The value for DeltaE depends on your choice in Athena. 2) You have to decide if you original choice of Ezero (in Athena) was close enough. If deltaE (from the Artemis fit) is more than +/- 10eV you need to go back into Athena move Ezero and redo step 1. The rule is: deltaE + Ezero should be somewhere on the edge of the xmu data. 2b) If you can not get a reasonable deltaE then rethink your theory. Maybe R in the theory is way wrong? Maybe two shells are needed instead of one? Or maybe Ezero in Athena is way off? Maybe your k-range needs to be even smaller. 2c) Still having trouble with step 1? --you can also change the k-weight for the background removal. For this first step it is often useful to crank it up, so that the higher k-region has a nice bkg and then just don't use the low k-region in the fit to get the theory to "line up" with the data. For example: use k=[4:12] in Artemis, with k-weight=3 in Athena for the background. 3) Now you have a theory for the first shell peak with an Ezero somewhere on the edge, we want to match-up the data and theory before the first shell peak. Go back into Athena, read in the fit in k-space. Then highlight your data, pull down the standard menu and select the theory data file. Redo the bkg by plotting both the theory and the data in k-space. They should look similar at low k. Then plot the FT (using the same parameters as you did in Artemis) and they should look similar below the edge. 3b) If not: --If they do not cross zero in k-space (at low k-values) at the same places you need to adjust Ezero. --If they do not have the same amplitude you may need to adjust Rbkg slightly. Remember Rbkg can not be greater than Kmin in for the data region used in Artemis, but they can be equal and often are equal. 4) Redo the fit and add some more shells as needed, increase the data range in k-space. See how much data you can use and still get reasonable fit and uncertainties. 5) At the very end you might want to repeat this procedure to make a pretty picture for your pub. HTH Shelly