[Ifeffit] Neagtive amplitude

Ravel, Bruce bravel at bnl.gov
Sun Feb 22 08:12:14 CST 2009

> Hi all,
>          I am fitting a *copper foil* . I am getting a good fit after using
> the *4 single scattering paths* but in case of *fourth path* I am
> getting *negative
> amplitude*. What should be the *probable reason* for that as I am getting a
> *very good fit*.


Multiple scattering is not optional ;-)

Look at the attached screenshot.  I have imported a feff calculation on copper metal into Artemis and plotted the fourth neighbor -- the one at 5.1 Ang. -- along with a few of the multiple scattering paths at the same distance.  The SS path is the blue one.  The plot is the real part of chi(R), which should help explain your result.

Note that the SS path and the various MS paths are not in phase.  When you include the MS paths in the fit, there is some complicated phase relationship between them such that they add up to equal the measured EXAFS.  If you leave out the MS paths, you leave out a huge amount of information required to properly fit the data.

Note also that the SS path is NOT the largest of these paths.  Because the MS paths at that distance involve collinear atoms, the MS paths are effectively amplified by the large peak in the scattering amplitude in the forward direction.

The moral of the story is that you need to examine ALL the paths before deciding which ones you can neglect in your fit.


 Bruce Ravel  ----------------------------------- bravel at bnl.gov

 National Institute of Standards and Technology
 Synchrotron Methods Group at Brookhaven National Laboratory
 Building 535A
 Upton NY, 11973

 My homepage:    http://cars9.uchicago.edu/~ravel
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/

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