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