FeII

Fe II refers to the singly ionized iron ion, Fe+, the iron atom after it has lost one electron. It has atomic number 26, and in its ground state the electron configuration is typically described as [Ar] 3d6 4s1, with a ground-term that gives rise to a rich spectrum of lines. The Fe II ion exhibits an extensive array of electronic transitions, leading to thousands of absorption and emission lines across the ultraviolet, optical, and near-infrared regions.

The Fe II spectrum is characterized by numerous multiplets arising from transitions among many excited levels, making Fe II one of the most prominent heavy-element tracers in astronomical spectra. In practice, Fe II lines appear prominently in the UV, such as common multiplets around 2380 Å and 2600 Å, and in optical regions in active galaxies, quasars, and certain stellar environments. The complexity of its spectrum reflects the availability of many metastable levels and the sensitivity of line strengths to density, temperature, and the radiation field.

In astrophysics, Fe II is used as a diagnostic of physical conditions in stars, the interstellar medium, and the circumgalactic and intergalactic medium. Its lines help determine metallicity, ionization balance, and gas dynamics. Ratios involving Fe II, Fe I, and other ions (notably Mg II) provide insights into chemical evolution and star-formation histories, since iron production is tied to Type Ia supernovae on longer timescales compared to α-elements like magnesium.

Laboratory measurements and theoretical atomic data for Fe II underpin the interpretation of astronomical spectra, enabling abundance determinations and the study of cosmic chemical evolution across local and distant environments.