Rotational Rest Frequencies for FeN (X²Δ_i) and Revised Spectroscopic Constants for FeC (X³Δ_i)

The pure rotational spectrum of FeN in its X²Δ_i ground state has been recorded using millimeter-wave direct absorption techniques in the range 198-525 GHz. New measurements have also been carried out for FeC (X³Δ_i), in particular of the Ω = 1 fine-structure component and the ⁵⁴FeC isotopomer. These molecules were created by the reaction of iron vapor with either CH₄ (FeC) or N₂ (FeN) in a DC discharge. Eight rotational transitions were recorded for FeN in its lowest-lying spin component, Ω = 5/2, and multiple transitions were measured for FeC in all three of its spin-orbit ladders, as well as for ⁵⁴FeC (Ω = 3). These data have been analyzed, and precise spectroscopic constants for both radicals have been determined. The fine structure in FeC was found to exhibit an irregular pattern, indicating that higher order spin-orbit perturbations are occurring in this molecule. Although only one spin component was observed for FeN, the bond length established from the Ω = 5/2 data is consistent with a ²Δ ground state, as indicated by theory. Fe-bearing species are relevant to many astrophysical topics, including astrochemistry, dust grains composition, nucleosynthesis, and mass loss from asymptotic giant branch stars.