13.4. SSPath object

The SSPath, or “single scattering path”, is, like the FSPath, derived from the Path object and so can be used just like a Path in a fit or a plot. Unlike a Path object, which is associated with a particular scattering geometry from the cluster of atoms used in the FEFF calculation, the SSPath is a generalized single scattering path computed using a particular scattering potential from the FEFF calculation, but with the scattering atom in an arbitrary location and at a specified distance.

The problem this is intended to solve is one for which the FEFF calculation is only an approximation of the actual structure. In this case, one purpose of the EXAFS analysis is to determine the extent to which the measured structure differs from the model structure. The paths used in the fit are parameterized to somehow determine these differences.

Sometimes, in the course of pursuing the analysis project, it is useful to examine the prospect of of a particular scattering atom at a distance that does not exist in the model structure. In practice and without DEMETER, this is kind of tricky. You must either find a different model structure which does have the desired atom at approximately the desired distance or have detailed knowledge of the inner workings of FEFF to know how to generate theory for that trial scatterer.

The SSPath object makes this data analysis trick very easy. In an existing FEFF calculation and its associated Feff object, a set of scattering potentials have been defined and calculated. In the feff.inp file, these are the ipots, or unique potential indices, in the POTENTIALS list.

To define an SSPath, you must specify the Feff object, the potential index of the desired scatterer, and the distance. Here is the SSPath used in the example of the uranyl ion in solution.

my $sspath = Demeter::SSPath->new(parent => $feff,
                                  data   => $data,
                                  name   => "hydration sphere",
                                  ipot   => 3,
                                  reff   => 3.5,
                                 );

As you can read in the section on that example, the SSPath is used to model the possibility of scattering from the oxygen atoms in the hydration sphere around the ion in solution. The arguments to the new method shown above are the bare requirements for creating an SSPath. In this case, unique potential 3 is the equatorial oxygen atom. Thus, the approximation is made that the scattering potential of the existing kind of oxygen atom is enough like this test scatterer that its potential can be used.

Once created (or, indeed, as arguments during creation, the SSPath is parametrized exactly like any normal Path, i.e.:

$fspath -> set(s02 => 'Nhydr * amp', enot=>'e0hydr');

Once created, the SSPath is used in a plot or a fit just like a normal Path object.

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