On 07/27/2016 04:39 PM, Matthew Marcus wrote:
I'm not sure I get your point about ion-chamber response. Shouldn't that normalize out in post-edge spline?
Athena edge-step normalizes (i.e. normalizes by a constant) by default. It does not do a functional (or energy dependent) normalization by default in the way you are suggesting.
If the I0 chamber gets less sensitive as a function of energy, then the post-edge background rises by the same amount as the wiggles get bigger, so it divides out. Similarly for probe-depth effects, depending on concentration. Another couple of possible sources of amplitude error: 1. The model for the pre-edge background is inaccurate, so that its extrapolation to the EXAFS region is incorrect. For instance, if there's a lot of elastic scatter getting into the fluorescence detector, then the pre-edge is curved, and if you model that with a straight line fitted near the edge, them you'll be off far from the edge. Similarly, in a long transmission scan, both pre- and post-edge are curved. My background-sub program has simple models for thse shapes which aren't perfect but help. This effect is really hard to control for because you have no real way to know what the 'right' answer is, most of the time.
2. Overabsorption will definitely reduce the amplitude of the wiggles, and I'm surprise Bruce didn't menntion it. It looks much like pinhole effect. Harmonics do similar things as well in transmission.
My brain is really tiny and it's getting to be late in the afternoon here. It's amazing that many bullet points actually made it into the email! B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Science Group at NSLS-II Building 743, Room 114 Upton NY, 11973 Homepage: http://bruceravel.github.io/home/ Software: https://github.com/bruceravel Demeter: http://bruceravel.github.io/demeter/