In the close coupling (CC) approximation the total wave function of the (e + ion)
system
is represented as
where 
is the antisymmetrization operator,
is the target ion wave function in a specific state
,
is the wave function for the free electron,
and 
are short range correlation functions for the bound
(e + ion) system.
Accurate computations for Fe I must allow for
channel couplings among many states of
the
target ion Fe II for at least three reasons. First, the
configuration interaction (CI) between the numerous low lying LS terms
is important. Second, it is known that strong dipole couplings between
opposite parity terms within the target ion give rise to large
photoexcitation-of-core (PEC) resonances at corresponding incident
photon frequencies (Yu & Seaton 1987). Third, the photoionization
channels
corresponding to the
ionization of the open inner 3d shell are likely to contribute considerably
to the total cross section. This requires that terms dominated
by the configurations 
and 
of Fe II should also be
included.
The photoionization of Fe I is considered as
Table 1: Target terms and correlation configurations for Fe II. The
values of the scaling parameters 
for
each orbital in the Thomas-Fermi-Dirac potential used in Superstructure are also
given
Table 2: Calculated (cal) energy levels of Fe II and comparison with
observed (obs) levels from Sugar & Corliss (1985). The energies (in Rydberg) are relative to the
ground state
Table 3: Correlation functions for Fe I included in the CC expansion.
Upper panel: correlations for quintets and singlets; lower panel:
correlations for singlets and triplets
A total of 52 LS terms of the target ion Fe II were included in the
close-coupling (CC) expansion for this calculation. Table 1
lists all the coupled LS terms in the
present calculations, as well as the correlation configurations included
for CI. The atomic structure program SUPERSTRUCTURE
(Eissner et al. 1974; Nussbaumer & Storey 1978) was used
to optimize the target wavefunctions. Table 2 compares the calculated
energies for the 52 terms of Fe II included in the calculation. In
general, the target energies obtained were within 10% of the experimentally
observed energies for the 52 terms, with few exceptions.
A further indication of the accuracy
of the target wavefunctions was the good agreement 
between the
length
and the velocity dipole oscillator strengths between opposite parity terms
of Fe II.
