The aim of this paper is to calculate low-temperature electron excitation data for atomic iron (Fe I). This involves establishing the near-threshold behaviour of the collision strength, a challenging problem in theoretical atomic physics.
Fe I is a particularly interesting and testing case. It is present in many types of cool or cold plasma, e.g. laboratory-produced plasmas, K-type and M-type stars, the interstellar media, and supernovae remnants such as SN 1987A where Fe I infrared emission lines have been observed (Li et al. 1993). The difficulties for the atomic theoretician are that many effects have to be taken into account at low energies, e.g. fine-structure, long-range polarization, and channel coupling between nearby states. A further difficulty is associated with computing the algebra for such a complex open d-shell atomic system.
Consider the first three terms in Fe I
(
, 
, 
),
whose energies span the range up to 0.12 Ryd. above the 
ground state.
This is clearly the minimum number of terms that need to be included
in a close-coupling formulation,
in order to obtain effective collision strengths over a reasonable
temperature range (say up to 4000 K, 
).
However, data are also required for applications at much lower temperatures (below 1000 K), comparable to the fine-structure splittings. At very low electron energies, large radial distances play an important role, and long-range polarization effects are important (Berrington 1988). Also the spin-orbit interaction is strong in the collisional Hamiltonian, and a proper treatment must account for the kinematics of the scattering electron in the different fine-structure channels.
This work is part of an international collaboration known as the IRON Project (Hummer et al. 1993, referred to as Paper I) to obtain accurate collision rates for fine-structure transitions. A full list of these papers published to-date is included in the references.