NAME¶

Xray::Absorption::CL - Perl interface to the Cromer-Liberman tables

SYNOPSIS¶

   use Xray::Absorption;
   Xray::Absorption -> load("cl");

See the documentation for Xray::Absorption for details.

DESCRIPTION¶

This module is inherited by the Xray::Absorption module and provides access to the data contained in the Cromer-Liberman tables of anomalous scattering factors and line and edge energies.

The data in this module and the Fortran code which it calls as a shared library, referred to as "The CL Tables", was published as

  S. Brennan and P.L. Cowen, Rev. Sci. Instrum, vol 63,
  p.850 (1992)

More information about these data is available on the Web at

   http://www.slac.ssrl.stanford.edu/absorb.html.

The values for the anomalous scattering factors are calculated by calls to the Ifeffit library by Matt Newville.

The values of edge and line energies are contained in a database file called cl.db which is generated at install time from the flat text files of the these data. The data is stored in a Storable archive using "network" ordering. This allows speedy disk and memory access along with network and platform portability.

The required "File::Spec", "Chemistry::Elements", "Storable" are available from CPAN.

METHODS¶

The behaviour of the methods in this module is a bit different from other modules used by "Xray::Absorption". This section describes methods which behave differently for this data resource.

"get_energy"

Example:

   $energy = Xray::Absorption -> get_energy($elem, $edge);
    

This behaves similarly to the "get_energy" method of the other resources. When using the CL data resource, $edge can be any of K, L1-L3, M1-M5, N1-N7, O1-O7, or P1-P3. Line energies are not supplied with the CL data set. The line energies from the McMaster tables are used.

"cross_section"

Example:

   $xsec = Xray::Absorption -> cross_section($elem, $energy, $mode);
   @xsec = Xray::Absorption -> cross_section($elem, \@energy, $mode);
    

For this data resource, one call in list context is considerably faster than repeated calls in scalar context. It is well worth the trouble of organizing your code to make a single call in list context and store the results for later use.

This behaves slightly differently from the similar method for the McMaster and Elam resources. The CL tables are actually tables of anomalous scattering factors and do not come with coherent and incoherent scattering cross-sections. The photo-electric cross-section is calculated from the imaginary part of the anomalous scattering by the formula

     mu = 2 * r_e * lambda * conv * f_2
    

where, "r_e" is the classical electron radius, lamdba is the photon wavelength, and conv is a units conversion factor.

     r_e    = 2.817938 x 10^-15 m
     lambda = 2 pi hbar c / energy
     hbar*c = 1973.27053324 eV*Angstrom
     conv   = Avagadro / atomic weight
            = 6.022045e7 / weight in cgs
    

The $mode argument is different here than for the other resources. The options are "xsec", "f1", and "f2", telling this method to return the cross-section or the real or imaginary anomalous scattering factor, respectively.

The values for f1 and f2 are computed by linear interpolation of a semi-log scale. Care is taken to avoid the discontinuities at the edges.

Because the CL tables do not include the coherent and incoherent scattering terms, the value returned by "get_energy" is a bit smaller using the CL tables than using the others.

EDGE AND LINE ENERGIES¶

The CL data resource provides a fairly complete set of edge energies. Any edge tabulated on the Gwyn William's Table of Electron Binding Energies for the Elements (that's the one published by NSLS and on the door of just about every hutch at NSLS) is in the CL data resource. The CL data comes with the same, limited set of fluorescence energies as McMaster.

BUGS AND THINGS TO DO¶

None that I know about...

AUTHOR¶

  Bruce Ravel, bruce@phys.washington.edu
  http://feff.phys.washington.edu/~ravel/software/Absorption