  The emission spectra and the ionization structure of the low ionization
stages of iron, Fe I--IV, in gaseous nebulae are studied. This work includes:
(i) new atomic data: photoionization cross sections, total e-ion recombination
rates, excitation collision strengths, and transition probabilities; (ii)
detailed study of excitation mechanisms for the [Fe II], [Fe III], and [Fe IV]
emission, and spectroscopic analysis of the observed IR, optical, and UV
spectra; (iii) study of the physical structure and kinematics of the nebulae
and their ionization fronts. Spectral analysis of the well observed Orion
nebula is carried out as a test case, using extensive collisional-radiative and
photoionization models. It is shown that the [Fe II] emission from the Orion
nebula is predominantly excited via electron collisions in high density
partially ionized zones; radiative fluorescence is relatively less effective.
Further evidence for high density zones is derived from the [O I] and [Ni II]
spectral lines, as well as from the kinematic measurements of ionic species in
the nebula. The ionization structure of iron in Orion is modeled using the
newly calculated atomic data, showing some significant differences from
previous models. The new model suggests a fully ionized H II region at
densities on the order of $10^3$ cm$^{-3}$, and a dynamic partially ionized H
II/H I region at densities of $10^5-10^7$ \cm3. Photoionization models also
indicate that the optical [O I] and [Fe II] emission originates in high density
partially ionized regions within ionization fronts. The gas phase iron
abundance in Orion is estimated from observed spectra.
