On Oct 14, 2010, at 7:39 AM, Matt Newville wrote:

If you're just getting started, I would say to not worry about energy alignment until it becomes an obvious problem.

A cautionary tale (with details made up, since I don't remember them!) from when I was just starting out as to what constitutes an "obvious problem": I collected five transmission EXAFS scans on the same sample. The scans were on top of each other when I looked at the graph, so I merged them...and proceeded to get somewhat screwy fits. The problem? I only looked at the graph across the whole spectra--say, 1500 eV. It turns out there was about a 0.7 eV shift between each scan and the next one, for a total of roughly 3 eV . That was small enough so as to be invisible when looked at on that scale. When I looked at just the XANES, though, the shift did become "obvious." I aligned the spectra and merged them, and suddenly the problems in the fit went away! Since then, I've seen the same thing happen with students to whom I am teaching the technique. On the other hand, there's no magic "blessing" given by the process of alignment. Suppose I have ten scans of very noisy data, and no reference. If I used the auto-align procedure in Athena, it sometimes shifts a scan 0.3 eV one way, sometimes 0.2 eV the other way, with no apparent rhyme or reason. Looking at the graphs, even zoomed in, just shows a bunch of noisy data roughly on top of each other. In that case, there's no reason to believe there are actual shifts between scans, and I would NOT align them prior to merging. Finally, beamline scientists usually have a very good idea whether their line is prone to drifts. Ask them! --Scott Calvin Sarah Lawrence College