[Ifeffit] bond distance resolution and correlation of parameters in MS analyses?

Bruce Ravel bravel at anl.gov
Wed Jun 6 07:47:01 CDT 2007


On Wednesday 06 June 2007, MarkBondin wrote:
> 1)Does the equation for bond distance resolution (r = pi/2deltak) only
> apply to SS? I have held the opinion that this can be applied to MS
> analysis however I have recently been informed that this equation does not
> correctly describe distance resolution in MS analyses. The paper in Coord.
> Chem. Rev. 2005, 249, 141-160 describes this and is this concordant with
> the views of the wider EXAFS community?


Mark,

Strictly speaking, that equation doesn't have anything to do with
EXAFS.  That is the equation that tells you what your Fourier
component resolution is in a general Fourier analysis problem.  It
just so happens that, in the case of single scattering EXAFS analysis,
that equation is easily interpreted in terms of photoelectron
wavenumber k and SS path length r.

The equation is neither different nor incorrect for MS analysis.
That's true becuase MS analysis isn't any different from SS analysis.
In either case, you do a Fourier transform.  In either case, you
attempt to model Fourier components using the contributions from some
number of scattering geometries as computed by theory.  In either
case, you are asking yourself if you can actually resolve small
differences in phase of the various things that contribute to the fit.

The only difference lies in how you *interpret* the physical meaning
of the Fourier components.  And even then, things aren't so very
different.  In the case of SS analysis, you assert that the R axis is
a measure of "bond length" while for MS analysis the R axis is a
measure of "half path length" -- acknowledging, of course, that there
is a phase shift in the EXAFS equation such that the R axis actually
measures something a bit shorter than the bond or half path length.

Off the top of my head, I don't recall the paper you cite and I most
certainly cannot speak for the "wider EXAFS community".  Speaking for
myself, the physical interpretation of the equation for Fourier
component resolution may change when you consider MS paths, but to
claim that a property of the Fourier transform somehow becomes invalid
when you change the details of the fitting model is just silly.

B


-- 
 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

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