[Ifeffit] A basic question about data collection

[Kropf, Arthur Jeremy]

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 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 
> 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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