On Tuesday 20 April 2004 11:56 pm, Barton, David (DG) wrote:
One final question. I often see a strong discontinuity near the edge energy in the background subtraction. The attached Athena project clearly shows this discontinuity, any thoughts about this?
Dave, I am CCing the answer to this one to the mailing list. The background spline is evaluated between the energies (or k values) specified. Outside that range the spline is not evaluated at all. To make the arrays conatining the data and background spectra easier to handle when interacting directly with Ifeffit, Matt wisely chose to make the background array the same length as the data array. At energies within the spline range, the background array contains the background spline. Outside the spline range, that background array contains the original data. Not only does this make the arays the same length, but it makes computation of chi(E) completely trivial -- you can just subtract the background array from the data array point by point. If the first or last few points of the spline are not near in value to the data (which can happen since a spline can be poorly constrained at the ends), then the plot of the data and background arrays will be as you described. The background array will have a discontinuity, but that is not a bug. Regardless of the constraint of the spline near its ends, the criterion for choosing the spline is met. That is, the spline is always chosen to minimize the low-R Fourier components (possibly with respect to a standard spectrum). Sometimes this minimization results in a spline that deviates from the data at the ends. Constraining that deviation is the purpose of the spline clamps.
I typically have ignored this since it is outside the EXAFS fitting region and the XANES background subtraction routines are not affected by it.
That is completely correct. HTH, B -- 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/