Molecular beam rotational spectroscopy of the<i>X</i>⁵Δ ground state of FeO

Abstract

Pure rotational transitions in the three lowest multiplet levels Θ = 4, 3, and 2 of the X 5Δ i ground state of FeO were observed using the method of microwave optical double resonance with separated optical and microwave fields on a collimated molecular beam. The spectra were observed with linewidths of 50 kHz in a frequency range between 120 GHz and 430 GHz, which corresponds to a range of rotational quantum numbers between 3 and 14. These highly precise data can still be described by a simple effective Hamiltonian. The molecular parameters derived are more accurate by a factor of five than those reported in the literature. But the parameters represent sums over different rotational electronic interactions and their interpretation needs electronic structure calculations for states as complex 5Δ i in FeO.