Millimeter-wave spectroscopy of FeF (X ⁶Δ_i): Rotational analysis and bonding study

The pure rotational spectrum of the FeF radical in its ⁶Δ_i ground electronic state has been recorded using millimeter/submillimeter direct absorption techniques. Transitions arising from all six spin orbit components have been observed in the v=0, 1. and 2 vibiational levels of ⁵⁶FeF, the main isotopic species, and also in the less abundant ⁵⁴Fe isotopomer. Hyperfine splittings, arising from the ¹⁹F nuclear spin of I=1/2, were resolved in the majority of transitions recorded, and lambda-doubling interactions were observed in the Ω=3/2. 1/2. and -1/2 spin-orbit ladders. The complete data set has been analyzed using a ⁶Δ Hamiltonian, and rotational, spin-orbit, spin-spin lambda-doubling, and hyperfine constants determined. This study has conclusively demonstrated that the ground electronic state of FeF is ⁶Δ_i. It also suggests that FeF has more covalent character to its bonding than alkaline earth or alkali metal counterparts.