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New Fe I Level Energies and Line Identifications from Stellar Spectra. III. Initial Results from UV, Optical, and Infrared Spectra

The spectrum of neutral iron is critical to astrophysics, yet furnace laboratory experiments cannot reach many high-lying Fe I levels. Instead, Peterson & Kurucz (2015) and Peterson, Kurucz & Ayres (2017) turned to 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 a thousand of these. An energy value was assumed for each unknown Fe I level, and confirmed if it shifted the predicted positions in updated Kurucz (2011) Fe I calculations to match exactly in wavelength the positions of four or more unidentified lines in the observed spectra. Nearly all these identifications were for LS levels characterized by spin-orbit coupling, whose lines fall primarily at UV and optical wavelengths. This extension of these searches provides nearly a hundred new Fe I level identifications. Thirty-nine LS levels are identified largely by incorporating published positions of unidentified laboratory Fe I lines with wavelengths < 2000A. Adding infrared spectra provided sixty Fe I jK levels, where a single isolated outer electron orbits a compact core. Their weak, blended lines fall mostly in the infrared, but are searchable because their mutual energies obey tight relationships. For each new Fe I level, this work again provides and makes publicly available its identification, its energy, and a list of over 16,000 of its potentially detectable lines with theoretical gf values. For over 2,000 of these, this work also includes astrophysical gf values, ones adjusted semi-empirically to fit the stellar spectra. The potential impact of this work on modeling UV and IR stellar spectra is noted.