The millimeter-wave spectrum of FeO in its X5Δi state (v = 0): a study of all five spin components

New millimeter/sub-millimeter pure rotational spectroscopy has been carried out for the FeO radical (X5Δi). The molecule was produced by the reaction of N2O with iron vapor, generated using a Broida-type oven. In addition to verifying previous measurements on FeO in the lower spin-orbit components, transitions originating in the Ω = 0 and 1 ladders have been recorded, for which lambda-type doubling was resolved. Spectra of the 54FeO isotopomer were also measured. The data set was analyzed successfully with a 5Δ Hamiltonian, demonstrating that highly perturbed, complicated electronic ground states can be modeled with effective rotational parameters.