Laboratory millimeter-wave spectrum of iron monoxide, FeO

Laboratory observations have been conducted at 154 and 184 GHz for FeO's pure rotational spectrum in the 5Delta-i ground electronic state, generating the radical in a 1-m long, free-space absorption cell by means of a DC glow discharge in a mixture of ferrocene and oxygen. Two rotational transistions of J = 5 - 4 and 6 - 5 in each of the three lowest spin sublevels with Omega values of 4, 3 and 2 were precisely measured, and the frequencies obtained were subjected to least-squares analysis using a Hamiltonian appropriate for the 5Delta-i electronic state.