Dear Shelly, Dear Bruce Thanks for both your answers. Going through Shelly's slides was a nice refresher course. Bruce, ok, I see why normalization is not affected. However.... The data were already correctly aligned. I've now explicitly tied the data to the standard, using the Data menu, and the auto-align shift was only 0.02 eV. So this had no effect on the spike. Setting the lower bound of the spline range to 0 mainly shifts the position of the spike to near k=0, and so hides it somewhat when we multiply chi by a power of k. But it's still visible in E space (a large negative peak in the spline). Setting a spine clamp at the low end changes things slightly but not qualitatively. Changing E0 would seem to be a reasonable step. After all, there is no reason why k=0 should coincide exactly with the first inflection point on the edge!! But again, all attempts, for Eshift varying from -5 to 10eV, make no qualitative difference. All other tweaks I've tried also maintain the spike, or else distort the spline so much as to introduce other errors. For comparison, I took the first curve (labeled 0.12) in project AuChloride_LCF_example.prj from Scott's talk. There is something of a sharp departure between the spline and the data visible in E space, as in my case. But the plot in k space -- even of chi itself without multiplying a power of k -- is nice and smooth. All this is in http://www.msri.org/~levy/files/example.prj which contains only one group from me and one from Scott. Would you be willing to take a closer look? I'd not be asking except that I'm completely stumped. My data seem perfectly normal. Thanks again, Silvio
Silvio, I think you should take a step back. Instead of asking yourself what you *want* your data to look like, you should ask yourself how the algorithms used actually work. The background removal is done using a spline: http://cars9.uchicago.edu/~newville/autobk/ProgramDoc/ Splines, by their very nature, have limited ability to conform to an arbitrary shape. By restricting the spline to use a number of knots determined by the maximal information content of the region between 0 and Rbkg, the spline can only have those Fourier components (neglecting leakage due to the finite transform range). One result of the limited Fourier content of the spline is a difficulty following the most swiftly varying parts of the data. Typically the most swiftly varying part of the data is the bit near the edge. That has the additional issue of being the beginning of the data range. Consequently, that end of the spline is only constrained to follow the data on the higher energy side. The result of this is that the spline sometimes fails to follow the data near the edge -- at least it fails to follow it in the same way that you would draw the background with a crayon. It does a perfectly fine job of optimizing the low-R Fourier components. So what is the impact of this spike in the background function? Well, instead of gnashing our teeth, let's check it out. Make a copy of the HGSREDPOW_026.001 group. For both of them use the same background removal parameters you currently have in your project file. Also for both of them, set the Fourier tranform dk value to 0 -- that is the value used in the background removal. For the origin, set the FT k-range from 0 to 13.5 -- i.e. the entire k-range. For the copy, set the FT k-range to [1:13.5]. Overplot the two data as chi(R). They are not very different, even though the copy avoids using the "bad" region at low k in its FT. In any case, at the level of Athena, you don't actually have sufficient information to be critical of the fine details of the background removal. You can, I suppose, jigger the background removal parameters until you have "despikified" your data to your satisfaction. However, the real question is whether you can analyze and interpret your data and whether your fitting results are highly correlated to the background removal. Any thing else is merely aesthetics. B -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/
participants (2)
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Bruce Ravel -
Silvio Levy