2022ApJS..260...28P

The spectrum of neutral iron is critical to astrophysics, yet furnace laboratory experiments cannot reach high-lying Fe I levels. Instead, Peterson & Kurucz and Peterson et al. adopted ultraviolet (UV) and optical spectra of warm stars to identify and assign energies for 124 Fe I levels with 1900 detectable Fe I lines, and to derive astrophysical gf values for over 1000 of these. An energy value was assumed for each unknown Fe I level, and confirmed if the wavelengths predicted in updated Kurucz Fe I calculations matched the wavelengths of four or more unidentified lines in the observed spectra. Nearly all these identifications were for LS levels, those characterized by spin-orbit coupling, whose lines fall primarily at UV and optical wavelengths. This work contributes nearly 100 new Fe I level identifications. Thirty-nine LS levels are identified largely by incorporating published positions of unidentified laboratory Fe I lines with wavelengths <2000 Å. Adding infrared (IR) spectra provided 60 Fe I jK levels, where a single outer electron orbits a compact core. Their weak IR lines are searchable, because their mutual energies obey tight relationships. For each new Fe I level, this work again makes publicly available its identification, its energy, and a list of its potentially detectable lines with theoretical gf values, totalling >16,000 lines. For over 2000 of these, this work provides astrophysical gf values adjusted semiempirically to fit the stellar spectra. The potential impact of this work on modeling UV and IR stellar spectra is noted.