On Thursday 19 February 2004 08:17 pm, Peter Southon wrote:
I'm wondering what principles you have to determine the appropriate k-weight to use for FTs. Until now I've used guidelines from a textbook by Teo, based on the Z of the backscattering atoms: Z > 57; kw = 1 57 > Z > 36; kw = 2 Z < 36; n = 3
I have a couple of things to add to what Shelly and Scott have already said on the k-weight topic. I think it is important to understand the rationale behind the guidelines Peter quotes from Teo. Atoms with low Z number have backscattering amplitudes (i.e. the F(k) function which Ifeffit reads from the feffNNNN.dat file) that have a peak at low k and tail off to almost nothing at high k. For instance, O, N, and C backscattering pretty much dies out above k=10. On the other hand, atoms with high Z numbers have rather small baskscattering amplitude at low k, but their scattering carries on to extremely high k. As an example, see Rev. Sci. Inst. v.71 #6 (2000) p. 2422. In figure 9, we see oscillations on a rhodium foil K-edge spectrum out past 30 invAng. So, the point of Teo's guideline is to try to "even out" the oscillations. That is, to try to make the oscillations near the end of the fitted spectrum about the same size as the oscillations near the beginning. That way, every part of the data contributes to a fit evenly, in some sense. Thus, what Teo was really suggesting was to choose a k-weight that does this for your data. I think that is not seeing the whole picture, however, as both Shelly and Scott have already explained. As Shelly suggested, Matt has taken a slightly different attitude in Ifeffit (and Artemis, of course, follows that lead). Ifeffit allows you to choose the k-weight of your choice, but it also allows you to choose 2 or more k-weights for your fit. Thus the chi-square fitting metric includes a sum over k-weights as well as a sum over data points. (See section 5.1 of http://cars9.uchicago.edu/feffit/feffit.ps. Eqn 5.1, then, becomes a double summation.) In Artemis, this is done by clicking more than one k-weight button on the page with the data parameters. As a final point, there is no reason that the k-weight *must* be an integer. If you are itching to k-weight your data by pi or by the Euler-Mascheroni constant, Ifeffit and Artemis both allow you do that. HTH, B -- ********* PLEASE NOTE MY NEW PHONE, FAX, & ROOM NUMBERS ****************** Bruce Ravel ----------------------------------- ravel@phys.washington.edu Code 6134, Building 3, Room 405 Naval Research Laboratory phone: (1) 202 767 2268 Washington DC 20375, USA fax: (1) 202 767 4642 NRL Synchrotron Radiation Consortium (NRL-SRC) Beamlines X11a, X11b, X23b National Synchrotron Light Source Brookhaven National Laboratory, Upton, NY 11973 My homepage: http://feff.phys.washington.edu/~ravel EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/