Laser spectroscopy of FeO: Rotational analysis of some subbands of the orange system

Abstract

Extensive laser excitation spectra and rotationally resolved laser-induced fluorescence spectra have been recorded for the “orange system” of gaseous FeO in the wavelength regions 5790–6140 and 5580–5640 Å. Detailed rotational analyses have been performed for about 20 Ω′ substates lying between 16 350 and 18 550 cm −1. These are found to comprise a very severely perturbed 5Δ i excited electronic state with a bond length of about 1.69 Å (which is responsible for the parallel polarization of the electronic transition from the 5Δ i ground electronic state) and a large number of “extra” Ω substates with B′ values ranging from 0.38 to 0.50 cm −1, which almost certainly belong to high vibrational levels of lower-lying electronic states. Evidence about the natures of the “extra” states is confusing, however, with the 54FeO 56FeO isotope shifts apparently being in conflict with the patterns of vibrationally resolved laser-induced fluorescence. Every single Ω substate that has been analyzed shows rotational perturbations of varying severity. The density and magnitude of the rotational perturbations are quite exceptional for a diatomic molecule, and result in a new type of totally chaotic diatomic spectrum. There is a remarkable similarity to the visible spectrum of NO 2: in NO 2 the complications arise from the high density of perturbing ground state vibrational levels; in FeO there is a correspondingly high density of perturbing electronic states at lower energy. The great complexity of the FeO spectrum arises because the states are in an awkward intermediate spin-coupling case which still resembles Hund's case (a) but shows strong tendencies toward Hund's case (c) coupling.