The rotational spectrum of the FeD radical in its X (4)Delta state, measured by far-infrared laser magnetic resonance.

Transitions between the spin-rotational levels of the FeD radical in the v=0 level of the X (4)Delta ground state have been detected by the technique of laser magnetic resonance at far-infrared wavelengths. Pure-rotational transitions have been observed for the three lowest spin components. Lambda-type doubling is resolved on all the observed transitions; nuclear hyperfine structure is not observed. The energy levels of FeD are strongly affected by the breakdown of the Born-Oppenheimer approximation and cannot be modeled accurately by an effective Hamiltonian. The data are therefore fitted to an empirical formula to yield term values and g-factors for the various spin-rotational levels involved.