Hi Dan,

You can't exclude paths by pushing the R-range up to just short of the Reff for the path! This is for three reasons:

1) The EXAFS Fourier transform is not the radial distribution function, although it is "related" to the radial distribution function. One example of this is that the contribution from a path tends to be centered lower (maybe half an angstrom) in R-space than the Reff associated with the path.

2) Atoms vibrate, or have static disorder. That's what sigma2 is about, after all. This means that if the mean absorber-scatterer distance is x, there are many cases where it is somewhat less than x.

3) Technical effects having to do with taking a Fourier transform of a finite data range introduce additional broadening into the signal due to a given path, so that it extends well below and above its mean value.

So how do you know how high to go in R? There are many ways to decide. One is to include the paths that you don't want to worry about in the fit (e.g. the MS paths at 3.989). Then, when the fit is done, use Artemis to plot those paths. You can then visually see how far down in R they have a noticable effect, and set your Rmax accordingly. If you really want to be sure. run a fit with them included and one without. If the fit does not change significantly (R factor, parameters stay pretty stable), then you know you're OK. If the fit does change significantly, you've got to lower Rmax.

--Scott Calvin

Sarah lawrence College