NAME¶

Xray::Absorption::Chantler - Perl interface to the Chantler tables

SYNOPSIS¶

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

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 Chantler tables of anomalous scattering factors and line and edge energies. The data in this module, referred to as "The Chantler Tables", was published as

  C. T. Chantler
  Theoretical Form Factor, Attenuation, and Scattering Tabulation
     for Z = 1 - 92 from E = 1 - 10 eV to E = 0.4 - 1.0 MeV
  J. Phys. Chem. Ref. Data 24, 71 (1995)

This can be found on the web at

  http://physics.nist.gov/PhysRefData/FFast/Text/cover.html

The Chantler data is available on the web at

  http://physics.nist.gov/PhysRefData/FFast/html/form.html

More information can be found on the personal web page of C.T. Chantler

  http://optics.ph.unimelb.edu.au/~chantler/home.html

The data contained in a database file called chantler.db which is generated at install time from the flat text files of the Chantler 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", and "Storable" modules 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 Chantler data resource, $edge can be any of K, L1-L3, M1-M5, N1-N7, O1-O5, or P1-P3. Line energies are not supplied with the Chantler data set. The line energies from the McMaster tables are used.

"cross_section"

Example:

 

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

 

This behaves slightly differently from the similar method for the McMaster and Elam resources. The Chantler tables contain anomalous scattering factors and the sum of the 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", "f2", "photo", and "scatter" telling this method to return the full cross-section cross-section, the real or imaginary anomalous scattering factor, just the photoelectric crosss-section, or just the coherent and incoherent scattering, respectively.

 

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

EDGE AND LINE ENERGIES¶

The Chantler 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 Chantler data resource. The Chantler data comes with the same, limited set of fluorescence energies as McMaster.

BUGS AND THINGS TO DO¶

•

It would be nice to improve the inter-/extrapolation near absorption edges. As it stands, these tables produce really poor DAFS output.

AUTHOR¶

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