Scott, 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. Jeremy Kropf Chemical Engineering Division Argonne National Laboratory Argonne, IL 60439 Ph: 630.252.9398 Fx: 630.252.9373 kropf@cmt.anl.gov

-----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@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 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.

--Matt

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