A new data compilation of the cross section for the electron/photon collisions
with atoms/ions
Yukikazu Itikawa Institute of Space and Astronautical Science 3-1-1 Yoshinodai,
Sagamihara 229, Japan
Abstract
Electron or photon collisions with atoms and atomic ions are fundamental
processes in an ionized gas or a plasma. A new compilation of the cross section data
for those processes has been started as a project of international collaboration.
The processes considered include: elastic scattering, excitation, ionization, and
recombination. An evaluation of the data is the most important, but the most difficult,
task to do. It will be relied mainly on the expertise of the collaboraters of the project.
Electron scattering from atoms, or electron-impact excitation or ionization of atoms
or atomic ions, is an elementary process in an ionized gas or a plasma. To understand
any macroscopic phenomena in those gases, we need cross section data for those
collision processes. An equilibrium population of ion species in a plasma, for example,
is determined by the balance of ionization and recombination. To know the balance,
we need cross sections for those processes. To measure temperature and density of a
hot plasma (e.g., solar corona), we often detect spectra of X-ray or UV light emitted
from the plasma. This is based on the fact that the emission is caused by collisions
between plasma electrons and ions and hence it depends on the electron temperature.
To interpret the emission spectra, we need cross sections for, say, the electron-impact
excitation of atomic ions.
Gas discharges have many applications these days. They are used for lighting (e.g.,
fluorescent lamp), display (e.g., plasma panel display) and plasma fabrication (e.g.,
surface treatment). Energy distribution of electrons is a key quantity in those
discharges. It is determined essentially by the collision processes of the electrons with
other constituent particles. To model the discharge plasmas, people need to have cross
section data for the processes.
Photoabsorption or photoionization of atoms or atomic ions is also an important
process in various fields of application, such as astrophysics and laser science. In
many cases astrophysical plasmas are maintained by the ionization process with the
high-energy photons coming from near-by stars.
Cross sections for the electron or photon collisions with atoms or atomic ions can
even be regarded as a fundamental property of those atoms or ions. Photoinization
cross section is equivalent to the oscillator strength for the excitation of continuum
states. Cross sections of atoms and ions, in general, are intrinsically related to the
charge distribution or other atomic properties (e.g., dipole polarizability) of those
atoms or ions.
For electron-atom or photon-atom collisions, there have been several attempts to
make a comprehensive compilation of the cross section data. One of the earliest
examples is the Landolt-Bornstein, 6th Series, vol. I/1 "Atoms and Ions" (1950). In
1970s and 1980s, the Atomic Cross Section Data Center at the Joint Institute for
Laboratory Astrophysics (JILA), Colorado, U.S.A., was making its effort to compile
both the bibliographic and the numerical data on the electron/photon collisions with
atoms and molecules. They published results of their activity in various forms. For
instance, L.J.Kieffer, "Low-energy electron-collision cross section data" Part I:
Ionization, dissociation, vibrational excitation Atomic Data 1, 19 (1969); Part II:
Electronic-excitation cross sections, Atomic Data 1, 121 (1969); Part III: Total
scattering; differential elastic scattering, Atomic Data 2, 293 (1971). Unfortunately
the JILA Data Center has been closed now. Data compilation of the electron-impact
or photon-impact cross sections are occasionally reported from other groups, but they
are mostly for some specific application and the scope of them is often limited (for the
data compilations or data reviews in the field of atomic collisions, see the list
compiled by McDaniel et al [1], McDaniel and Mansky [2]). In other words, any
comprehensive and up-to-date compilation is unavailable at present. Here
"comprehensive" means that the compilation includes cross section data for any
possible processes involving atoms of any element at any collision energy.
Considering this situation, we have started a new comprehensive compilation of the
cross section data. This is an international effort. The result will be published as a new
volume of Landolt-Bornstein series by Springer. Numerical data are now being
collected for the processes
I. Photon collision with atoms
I.1. Photoionization cross section (total)
I.2. Photoionization cross section (partial)
I.3. Elastic and inelastic scattering
II. Electron collision with atoms
II.1. Total scattering cross section
II.2. Elastic scattering
II.3. Momentum-transfer cross section
II.4. Excitation
II.5. Ionization
III. Electron collision with atomic ions
III.1. Excitation
III.2. Ionization
III.3. Recombination
III.4. Detachment of electrons from negative ions
The most difficult part of our task is the evaluation of data. In many cases, available
data scatter widely. The difference in the cross sections measured by different groups
exceeds their error limits combined. As far as possible, we attempt to select one or a
few sets of data to be recommended for use. In most cases, we consider only the data
obtained experimentally. Theoretical results are consulted with only when needed for
evaluation of experimental data. However, very few experimental data are available
for electron-impact excitation of atomic ions. This is because it is very difficult to
prepare target ions abundant enough for the cross section measurement. Instead there
are a large number of calculations reported for the process. Exceptionally in this case,
theoretical values will be recommended to use. In any case, we need expertise to do
the evaluation. Considering this point, an expert in each category has been appointed
as an author for the respective chapter.
Another difficulty we encounter in the present project is the availability of the data.
Even for neutral atoms, the availability of the cross section data is not uniform. In the
case of photoionization, for instance, any reliable experimental data are available only
for 17 elements out of 92. Most measurements have been done for rare gas atoms (He,
Ne, Ar, Kr and Xe). This is because it is easy to prepare the target atoms. Though they
are very important in many fields, cross sections for reactive species (e.g., H, C, N,
O,...) are very scarce. In many practical cases, therefore, people have to rely on some
calculations or empirical formulas to get cross section data. We plan to include in our
compilation tables of theoretical values or simple empirical formulas, if they are
useful.
The resulting volume of the Landolt-Bornstein series will serve as a standard
reference not only for atomic physics but also for many application fields.
References
1. McDaniel, E.W., Flannery, M.R., Thomas, E.W., and Manson, S.T., Atomic
DataNucl. Data Tables 33, 1 (1985)
2. McDaniel, E.W. and Mansky, E.J., Adv. At. Mol. Opt. Phys. 33, 389 (1994)