[Ifeffit] Very large exafs amplitude in collected data - cause unknown

Matt Newville newville at cars.uchicago.edu
Wed Jun 5 16:47:49 CDT 2013

Hi Dan,

On Wed, Jun 5, 2013 at 1:18 PM, Daniel Olive <dolive at lbl.gov> wrote:
> Hi All,
> Well, this is certainly interesting. I had a chance to look at the data myself, and although I can't say if the results are meaningful, or useful, or even what you wanted, I don't see large amplitudes like Jason and Bruce report.
> A quick glance at the header of the files showed a list of column heading. Column 1 was energy. No I0, but, there was a counter, and everything else was DXP related so I guessed that was your ion chamber. DXP_Total caught my eye, and I'm guessing it sums the dxp## columns that follow. I loaded column 1 vs. (8/7) in Athena. I had to change all the groups to Au L3 edge, and I fixed the E0 to something sensible like 11923 (it's default position was somewhere after the white line). After changing the pre-edge range so it didn't end so close to the peak (-182 to -90) I merged them. k2 weighted graph attached.
> A quick first shell fit in Artemis gave me something like 15 Au neighbors at around the distance predicted in,
> http://prb.aps.org/abstract/PRB/v77/i7/e075432
> The plot even looks kind of like their data. (Disclaimer: this was by no means a proper fit, but not bad for 2 minutes and a google search).
> So...It may not be as hopeless as you think. Without knowing anything about the samples or the experimental setup, you may still need to look into dead time correction, self-absorption correction, etc., and by all means look at each detector channel and make sure you're not summing in bad channels. I can't agree strongly enough with the need to look at the data as it comes in (it's a lesson I suspect many of us learned the hard way at some point), and Jason, welcome to the list.
> Dan
> Daniel Olive     ---     dolive at lbl.gov
> Postdoctoral Researcher
> University of California, Berkeley
> Nuclear Science Division
> Lawrence Berkeley National Laboratory
> 1 Cyclotron Road
> MS 70A1150, Room 70A-2205B
> Berkeley, CA 94720-8175
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Thanks for that. I think many of would be interested to know what the
file really contains (there must be an I0 in there somewhere, no? but
if you just add channels multiple times, shouldn't that normalize out?
 how would it be k-dependent?).  It would be nice to straighten this
out, if for no other reason than to demonstrate that exceedingly large
and poorly documented data files can cause weird, difficult to
diagnose problem, even for the people who originally collected the


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