The orange system of (\mathrm{FeO}) has been reinvestigated using low-temperature molecular beam laser-induced fluorescence spectra, obtained by supersonic jet cooling. Two new weak bands have been found, and analyses of some of the previously known bands extended. Measurements of the ({ }^{54} \mathrm{Fe}-{ }^{56} \mathrm{Fe}) isotope shifts have been made for most of the bands, and the hyperfine structure of the low- (J) lines has been recorded for two of the strongest bands of ({ }^{57} \mathrm{FeO}). The isotope shifts are consistent with the presence of two ({ }^{5} \Delta_{i}-{ }^{5} \Delta_{i}) transitions lying within (1000 \mathrm{~cm}^{-1}); the origins of the (\Omega=4) spin components lie at 5583 and (6110 \AA), respectively. The hyperfine patterns and the spin-orbit structure indicate that the upper state electron configurations are ((3 d \delta)^{3}(3 d \pi)^{2}) ((3 d \sigma)^{1},\left(D^{5} \Delta_{i}, 5583 \AA\right)) and (\mathrm{O}(2 p \pi)^{3}(4 s \sigma)^{1}(3 d \delta)^{3}(3 d \pi)^{3},\left(D^{\prime} \Delta_{i}, 6110 \AA\right)). The bond length in the (D^{\prime}) state (\left(r_{0}=1.654 \AA\right)) has been obtained from a deperturbation of the (6110 \AA) band; it is only (0.035 \AA) longer than in the ground state, which indicates that electron promotion between the two (\pi) orbitals, nominally (\mathrm{O}(2 p \pi)) and (\mathrm{Fe}(3 d \pi)), has only a small effect on the strength of the bonding. The new isotope data still do not clarify the vibrational assignments of the higher levels, which are disorganized by extensive electronic perturbations. C) 1995 Academic Press. Inc.