NAME¶
Xray::Absorption::Henke - Perl interface to the Henke tables
SYNOPSIS¶
use Xray::Absorption;
Xray::Absorption -> load("henke");
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 Henke tables of anomalous scattering factors and
line and edge energies.
The data in this module, referred to as "The Henke Tables", was
published as
B. L. Henke, E. M. Gullikson, and J. C. Davis,
Atomic Data and Nuclear Data Tables Vol. 54 No. 2 (July 1993).
The Henke data is available on the web at
http://www-cxro.lbl.gov/optical_constants/ and more information about the data
can be obtained from Eric Gullikson <EMGullikson@lbl.gov>.
The data is contained in a database file called
henke.db which is
generated at install time from the flat text files of the Henke 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", 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 Henke 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 Henke data set. The line energies from the McMaster tables are
used.
- "cross_section"
- Examples:
$xsec = Xray::Absorption -> cross_section($elem, $energy, $mode);
@xsec = Xray::Absorption -> cross_section($elem, \@energy, $mode);
This behaves slightly differently from the similar method for the McMaster
and Elam resources. The Henke 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 Henke tables do not include the coherent and incoherent
scattering terms, the value returned by "get_energy" may be a
bit smaller using the Henke tables than that from the McMaster
tables.
EDGE AND LINE ENERGIES¶
The Henke 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 Henke data resource. The Henke 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/