[Ifeffit] A basic question about data collection

[scalvin at slc.edu]

Aha! So the reason I was taught to collect at k-space intervals of 0.05
A^-1 to avoid interpolation problems, NOT for the reason I thought at the
time. And since I've basically made a career so far out of concentrated
samples, it's been OK. But as long as I'm willing to throw in a binning
step, I'd use beam time more efficiently if I collected more closely
spaced data at high-k and less at low-k than I currently do. Or I can just
increase collection times at high k's relative to low k's.

Thanks, Matt, that clears things up!

--Scott Calvin
Sarah Lawrence College


>
> 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
>
>