On Monday 11 July 2005 17:28, Silvio Levy wrote:
Hi, I'm a new reader of the list. Thanks for all this very useful software and impressive documentation.
Thanks for the kind words.
I'm having trouble understanding the role of rbkg in spline (or autobk). Ravel's Exafs Analysis with FEFF and FEFFIT describes it as "the cutoff between the portion of the Fourier transformed spectrum dominated by the background function and the portion containing information about the local structure".
This would seem to imply that after removal of the background spline, the signal will have NO frequencies below rbkg -- that is, in the R domain, it would be 0 (or close to 0) for R < Rbkg.
That would be true if an infinite Fourier transform were possible. Alas, our FT is finite (no negative values of k and positive values only go out to 12 or so). Since the FT is finite, the Fourier components leak to the left and to the right. In practice, this means that our signal has non-zero spectral weight below Rbkg and our background has non-zero spectral weight above that value. Thus, in practice, the background spline and the parameters we eventually use to fit the data are correlated. Although there are many things in that document that I would state differently today, the language I used to describe Rbkg is valid. Rbkg *is* the frequency we choose as that cutoff, but the cutoff is never as clear as you anticipated. All the details of the Autobk algorithm are in Physical Review B47:21 (1993) p. 14126. See page 30 in this PDF file http://cars9.uchicago.edu/xafs/NSLS_2003/Kelly.pdf for a nice graphical demonstration of what Rbkg means. (The rest of that document is pretty useful, as well.) HTH, B -- Bruce Ravel ----------------------------------- bravel@anl.gov -or- ravel@phys.washington.edu *** My cell phone number has changed. Please ask if you need the new number Environmental Research Division, Building 203, Room E-165 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://feff.phys.washington.edu/~ravel EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/