Millimeter/submillimeter velocity modulation spectroscopy of FeO+ (X 6Σ+): Characterizing metal oxide cations

► Pure rotational spectrum of FeO+ measured using millimeter/submillimeter velocity modulation methods. ► First high-resolution spectrum of a metal oxide cation. ► Data suggest a low-lying 4Σ− state that could be involved in the catalytic activity of FeO+. ► Bond length provides evidence for covalent bonding in FeO+ and FeO.