[Ifeffit] A basic question about data collection

Matt Newville newville at cars.uchicago.edu
Thu Aug 25 15:34:22 CDT 2005

Hi Scott,

Yep, collecting with an even energy grid is essentially
k-weighting the collection by k^1.  It's not quite that simple
-- see below -- but it's close.  Of course, you can also step
evenly in k and increase the count time at each point.  A common
approach when usig solid-state fluorescence detectors and/or
dilute samples is to k-weight the collection time by k, k^2, or
k^3, and count for, say, 2sec per point at low-k and ramp up to
10sec per point at high-k (looking at a random recent scan).  
It does definitely help cut down the total collection time to
get reasonably clean spectra on dilute systems.

The challenge with using data that's on a fine energy grid is
that the routine converting energy to k has to know to use all
that data and also know *how* to use all that data. Typically
(at least, in ifeffit), data is interpolated from E to an
evenly-spaced k grid with a fairly simple interpolation scheme.  
If the energy data are too finely spaced, some data may actually
get ignored.  Collecting out to k=18A^-1 with 0.5 eV steps might
not work at smoothing out the data as well as you'd hope.  
Since k=18 -> E=1234.4 and k=18.05 -> 1241.3, there ~6eV between
adjacent k-points (assuming ifeffit's delta_k = 0.05).

This is important for QEXAFS (which typically does sample at a
very fine energy grid). I've been told by people doing QEXAFS
that a simple box-car average is good enough for binnning QEXAFS
data.  That's what Ifeffit's rebin() function does. I'd think
that a more sophisticated rolling average (convolution)  would
be better (and not screw up energy resolution), but apparantly
it's not an issue.


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