[Ifeffit] Re: Ifeffit digest, Vol 1 #60 - 1 msg

[Grant Bunker]

Hi, Matt - you might also add that if the energy resolution is small
enough (<3 eV say), differences in resolution don't affect the EXAFS,
because the EXAFS oscillates much more slowly than the energy width,
except perhaps right near the edge. Only in cases where the resolution
(or core hole lifetime) is really large (>5 eV)  will you see any noticeable
effect, and the effect is smaller the farther above the edge that you go.

The effect is always important for XANES though, and that can cause a
slight change in the EXAFS analysis. If the resolution is large for one
scan, and small for another, the detailed shape of the edge can be
altered. If the edge position is judged by the first derivative maximum or
some other metric of the edge shape, the nominal edge position could be
shifted an eV or two. This isn't an issue as long as people don't
overinterpret the edge positions, but it could confuse those who do.

Similar things happen with thickness effects and particle size effects and
self-absorption effects.

It is of course straightforward to compensate for spectral the differences
by convolution/deconvolution.

thanks - grant

On Mon, 30 Sep 2002 ifeffit-request at millenia.cars.aps.anl.gov wrote:

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>    1. experimental resolution (Matt Newville)
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> --__--__--
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> Message: 1
> Date: Mon, 30 Sep 2002 10:09:06 -0500 (CDT)
> From: Matt Newville <newville at cars.uchicago.edu>
> To: Irina Pirog <pirog at ip.rsu.ru>
> cc: ifeffit at millenia.cars.aps.anl.gov
> Subject: [Ifeffit] experimental resolution
>
> Dear Irina,
>
> On Wed, 25 Sep 2002, Irina Pirog wrote:
> >
> > Dear Dr. M. Newville. I'm a researcher of X-Ray department at
> > Physical Research institute of Rostov State university. In our
> > laboratory we use UWXAFS code treating experimental data. I will be
> > very gratitude to you, if you would answer me: how is experimental
> > resolution is taken into account in UWXAFS.
> >
> > Sincerely yours
> >
> > Irina Pirog.
>
> This is an excellent question that I'm sure other people may have, so
> I hope it's OK with you that I am responding to the Ifeffit mailing
> list, which we've set up to share such questions about EXAFS and
> Feff, Feffit, Ifeffit, etc.
>
> By itself, the UWXAFS programs (atoms, autobk, and feffit) don't know
> what the experimental resolution is.  In principle, the experimental
> resolution is important only to the extent that it is different
> between two measurements or between a measurement and a theoretical
> calculation.  For the UWXAFS programs, the only place this becomes
> important is when comparing Feff calculations to data in feffit.
>
> Generally speaking, Feff includes terms that effect the resolution
> and all other loss terms in the mean-free-path, lambda.  Feff
> includes contributions from the core-hole lifetime and some many-body
> electron loss terms in lambda, but it does not explicitly put in an
> experimental resolution.  But Feff usually has a little too much loss
> (ie, lambda is a little smaller than it should be), which turns out
> to be fine for experiments , with a resolution of ~1eV or so.
>
> But sometimes you know what the experimental resolution is and
> want to include it in the analysis.  To account for that,
> Feffit has an energy-broadening Path Parameter, ei, that gives
> an additional broadening term in the EXAFS equation through
>   p^2 = (k_real + i/lambda)^2 +   ei * 2m / hbar^2
>
> where k_real and lambda are taken from the feff0001.dat file for
> each path (though they are the same for any one Feff calculation),
> and then, the k and lambda terms used in the EXAFS equation are:
>   k      = Re[sqrt(p)]
>   lambda = 1/Im[sqrt(p)]
>
> To use this, you would have a feffit.inp with something like this:
>
>  Path   1  feff0001.dat
>  id     1  path 1
>  s02    1  amp_factor
>  ei     1  1.0            # add 1 eV broadening to Feff calc.
>  ...
>
> I mention the s02 parameter here because ei will be very highly
> correlated with s02 (and sigma2).  In fact, I'd guess that much of
> the variation in the observed s02 parameter for different
> measurements is actually due to differences in energy resolution.
>
>
> Finally, you may be interested in using the greatly revised versions
> of the UWXAFS programs, including graphical user interfaces for data
> processing and fitting:
>    http://cars.uchicago.edu/ifeffit
>
> Ifeffit also uses a Path Parameter called ei with the same effect,
> though the syntax would be slightly different:
>
>   path(1, file=feff0001.dat, s02=amp_factor,  ei= 1.)
>
> Anyway, I hope that answers the question,
>
> --Matt
>
> |= Matthew Newville        mailto:newville at cars.uchicago.edu
> |= GSECARS, Bldg 434A      voice: (630) 252-0431 / 1713
> |= Argonne Natl Lab        fax:   (630) 252-0436
> |= 9700 South Cass Ave     http://cars9.uchicago.edu/~newville/
> |= Argonne, IL 60439 USA
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