[Ifeffit] calibration/alignment

[Scott Calvin]

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