Laboratory detection of FeCO⁺ (X⁴Σ^-) by millimeter/submillimeter velocity modulation spectroscopy

The millimeter/submillimeter spectrum of the molecular ion FeCO⁺ (X⁴Σ^-) has been recorded using velocity modulation spectroscopy. The molecular ion was created in an AC discharge of Fe(CO) ₅ and argon. Twentyseven rotational transitions, each consisting of four fine-structure components, were measured in the range 198-418 GHz. The data were fit with a case b Hamiltonian, and rotational, spin-rotation, and spin-spin constants were determined. Because of the presence of higher order spin-orbit interactions, probably caused in part by a nearby ⁴II excited state, numerous centrifugal distortion terms were needed for the spectral analysis. The value of γ_s, the third-order spin-rotation constant, was also remarkably large at -72.4 MHz. Rest frequencies for FeCO⁺ are now available for interstellar and circumstellar searches. This species may be present in molecular clouds, where CO is abundant and gas-phase iron should be in the form of Fe⁺. Molecular ions such as FeCO⁺ could be the hidden carriers of metallic elements in such clouds.