On Monday 28 July 2003 02:48 pm, Peter Pfalzer wrote:
You're right. The "simple" correction with Corwin's approach will give a correction factor of (at least) the right order of magnitude with any detector. And as the selfabsorption doesn't do anything but smoothly reducing the EXAFS amplitude (at least if the sample's not too thin), one should be able to fit the remaining deviation with a slight change in S_0^2 - but keeping in mind that some cases could require more thorough treatment.
My two cents worth: Peter has hit the nail on the head in this paragraph. The point that the SA correction mostly affects the amplitude is indeed an important point. (The rest of the effect will be in sigma^2, also an amplitude term.) One can continue to measure bond length even *ignoring* SA corrections. It is not hard, though, to come up with a case where SA correction needs to be done well, though. Suppose, to make up an example from th top of my head, you are looking at some effect in the XANES in a series of ternary phase single crystals (i.e. something of the sort A_{1-x} B_x C). And suppose you are measuring the edge of the atom B. As x approaches 1, the SA correction becomes larger. The SA correction must be applied consistently to make meaningful conparisons between the data. So, any of the approaches out there in the literature (including doing nothing!) are possibly "adequate" if it is sufficient to measure the phase terms in the EXAFS equations. However, to try to make sense of a series of data sets, a consistent and stable SA correction is necessary. In that case it would be prudent for the experimenter to measure a toy system in which the SA correction is predictable to understand the limitations of any correction scheme. B -- Bruce Ravel ----------------------------------- ravel@phys.washington.edu Code 6134, Building 3, Room 222 Naval Research Laboratory phone: (1) 202 767 5947 Washington DC 20375, USA fax: (1) 202 767 1697 NRL Synchrotron Radiation Consortium (NRL-SRC) Beamlines X11a, X11b, X23b, X24c, U4b 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/