Re: [Ifeffit] pi/2deltak

Thank you very much for the explanations and your time. Best Wishes, -Riti

On Feb 1, 2016, at 9:16 AM, Robert Gordon wrote:

Hi Riti,

A couple of caveats to Matt's answer:

The EXAFS equation is not simply a sum of sine functions. I can think of two ways that the resolution criteria can be worked around:

1. Consider the case of two close near neighbours in a single-crystal environment whose close distances are geometrically distinct such that, if one were to conduct polarisation-dependent measurements, one could turn off one of the distances in one orientation (and perhaps the other in another), one could fit the two sets to extract the two close distances. (i.e. you aren't trying to resolve the two distances in one measurement)

2. If there is a pronounced chemical difference between the neighbours, they can be resolved even if the distance separation is small, provided the k-space range being used provides enough independent parameters to conduct the fit.

If you wish to convince yourself that these two caveats work, I suggest creating a model using the AuCu structure. If you contract or expand the c-axis, and substitute either Cu for Au (i.e. make Cu but in lower symmetry) or substitute a different atom on one of the Cu sites, you can test both Matt's explanation and my caveats above.

cheers, Robert

On 1/31/2016 6:33 PM, Matt Newville wrote:

...

Hi Riti,

On Sat, Jan 30, 2016 at 7:35 PM, Ritimukta Sarangi < mailto:ritimukta@gmail.comritimukta@gmail.com mailto:ritimukta@gmail.com> wrote: Hello,

I was recently asked about the accuracy of this formulation for obtaining EXAFS resolution and I did not have a good answer. Can someone point to a reference or explain here? Thank you for your time, Best, -Riti

The deltaR = pi / 2DeltaK follows from general Fourier analysis and formulas like it can be found in many signal processing textbooks. For on-line resources, googling "Frequency resolution Fourier transform" gives several good references.

The idea is that (using sound-waves as an example) in order to distinguish two close frequencies (say 440 Hz from 441 Hz, so a different of 1 Hz), you have to sample many periods (pi seconds) to be able to do this.

For EXAFS, if there are contributions from two neighbors that are very closely spaced, you would have to sample enough oscillations (go high enough in k) to see the effect of these two different distances beating against each other. If you don't go out far enough in k, you can't tell that these two contributions are actually from different distances.

Hope that helps,

--Matt

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