Low-lying energy levels of the FeH molecule

Abstract

High-resolution laser-excited fluorescence spectra of FeH at T ∼350 K have been recorded and analysed to derive new information about some of the low-lying electronic states. In particular, transitions assigned to e⁶Π—b⁶Π have enabled the lowest rotational levels of ν = 0 in b⁶Π_7/2, b⁶Π_5/2, and, with less certainty, b⁶Π_3/2, to be identified. The J = (7/2)± level of the ν = 0 level of the b⁶Π_7/2 state lies 3902.93cm⁻¹ above ν = 0, J = (7/2)± in X⁴ Δ_7/2. Another weaker series also appears to involve a state with a lowest J value of 7/2: in contrast to the behaviour of most other known states (except notably for X⁴Δ_7/2), the ΛA-type doubling in the rotational levels of the new state is very small, no more than 0.04cm⁻¹ at J = 11/2. This level appears to be ν = 0 of C⁴Φ_7/2, for which T ₀(7/2)± is determined to be 3755.28cm⁻¹. Even with the narrow-line excitation and relatively low temperature of the present experiments, fluorescence spectra are very rich and much analysis remains. However, the indications are that the ab initio calculations on the low-lying states give the right general impression, even if the predictions, especially for the quartet states, are not always correct in detail.