[Ifeffit] A basic question about data collection
Kropf, Arthur Jeremy
kropf at cmt.anl.gov
Thu Aug 25 16:05:09 CDT 2005
One often overlooked advantage of scanning in small energy steps is that
it is much easier to see if a small interfering absorption edge is
contaminating the data. Other than that, I agree with everything Matt
wrote. In the end it comes down to what is best for your particular
experiment; stepping through a fine energy grid may take prohibitively
long, if you must wait for the monochromator to settle for 1 second
between data points.
Chemical Engineering Division
Argonne National Laboratory
Argonne, IL 60439
kropf at cmt.anl.gov
> -----Original Message-----
> From: ifeffit-bounces at millenia.cars.aps.anl.gov
> [mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf
> Of Newville, Matthew G.
> Sent: Thursday, August 25, 2005 3:34 PM
> To: XAFS Analysis using Ifeffit
> Subject: Re: [Ifeffit] A basic question about data collection
> 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
> 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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