next up previous contents index
Next: References Up: 6 Examples Previous: 6.2 Further Examples

6.3 Suggestions

  Except for \ensuremath{E_0} and \ensuremath{R_{\rm bkg}} , I strongly suggest using the program defaults for the numerical parameters of autobk. Most importantly, do not use the same Fourier transform parameters that you would use for analysis (say, with feffit). The hard part of background removal is the low-k (or near-edge) part of the spectrum. You want to emphasize this region, by using a small k -weighting, like 1 (the default) or maybe even 0. Also, turn the Fourier Windows ``sills'' off, so that dk = 0. Otherwise the endpoints of the k -range will get no weight in the fit and \ensuremath{\mu_{0}(E)} will be unstable near the endpoints. Finally, start at low-k , like 0.00. If there is a strong white line you'll probably need to move \ensuremath{k_{\rm min}} to just past the white line, because the autobk background won't be able to follow most white lines.

The value of \ensuremath{R_{\rm bkg}} is the most important parameter, and the hardest to pick. Start with 1.0 Å or half the near-neighbor distance. You may then want to adjust it to a value where \ensuremath{\tilde\chi(R)} is a small fraction of the maximum of the \ensuremath{\tilde\chi(R)} for the first peak. Remember that the region you use for background-removal cannot be used in analysis. In fact the analysis of the first shell should begin on the next independent data point after \ensuremath{R_{\rm bkg}} , namely $R_{\rm bkg }+ \pi/\Delta k$, where $\Delta k$ is the k -range of the data. Don't go too far into the first shell or you won't have any data to analyze! A summary of suggested ranges for the most sensitive parameters in autobk is given in Table 3.


         

 
Table 3: Suggested values for a few important parameters in autobk. 
Keyword Suggested Range of Values Notes
Kmin 0.00 - 0.20 unless there is a ``white line''
Kweight 0 or 1 3 is probably too big
Dk   not bigger than 1.
Rbkg 1.0 or half Rnn keep $\\ gt \vert \tilde\chi(R_{\rm bkg}) \vert 
 \ll \, {\rm max}\{\vert\tilde\chi(R)\vert\}$


next up previous contents index
Next: References Up: 6 Examples Previous: 6.2 Further Examples

[ AUTOBK Home Page | AUTOBK Document | Matthew Newville ]