Hi Steven, I once published with k-ranges as small as 4-8. I think there are two related criteria you can look at off the bat to see if your k-range is definitely too small. One is, of course, the Nyquist criterion comparing independent points to variables being fit (automatically computed by IFEFFIT). This criterion assumes that information is "ideally packed" in the EXAFS signal, which it's not, so although fitting more variables than you have "independent points" according to this criterion is definitely bad, you really want to clear this criterion by a significant margin. The second (related) criterion is the resolution of different scatterers. A rule of thumb is that in order to resolve two different absorber-scatterer distances, you need to satisfy: kmax - kmin > pi/(2 x delta r) where delta r is the difference in absorber-scatterer distances you are hoping to resolve. You'll notice that with a k-range of 4-8, that means I couldn't resolve distances less than 0.4 angstroms apart... --Scott Calvin Sarah Lawrence College At 11:46 AM 4/27/2004 -0400, you wrote:
<fontfamily><param>Arial</param><smaller>Dear all, I always appreciate the big useful help from everyone. I have a basic question. I think a similar question has been arose long time ago. (But I lost the mail..) My sample has very small amount (under 1 wt%) and cluster size (<< 1nm). The spectrum is always noisy at high k range, so I use usually k=2 to 10 or 12. It depends on the noise of the spectrum. Of course, if I go further down, it is better. My question is how low can I set the k range? Is there any minimum range for reliable result? Please let me get out of this dilemma... Thank you always. Steven </smaller></fontfamily>__