Evidence for a Strong Intercombination Transition in FeH at around 510 nm

New rotational energy levels of the FeH radical have been discovered in the region between the ground X(4)Delta state and the a(6)Delta state some 1890 cm(-1) above it by the technique of dispersed fluorescence. The fluorescence was observed following excitation of the molecule to its e (6)Pi and g(6)Phi states. The dispersed fluorescence spectra from the former are complicated and those from the latter less so. The irregular nature of the dispersed fluorescence spectra is consistent with the observation of intercombination transitions and suggests that the levels belong to a state (or states) of quartet multiplicity. Two well-defined series of rotational levels are identified. The lower-lying one, with an energy origin 910 cm(-1) above the X (4)Delta (7/2) state and an Omega value of 5/2, is accessible from both e (6)Pi and g (6)Phi states. The second series. with an origin at 1068 cm(-1) and Omega = 3/2, is observed from the e (6)Pi and g (6)Phi states also. Both series show large A-type doublings. Although it is possible that these series form the two lowest spin components of the A (4)Pi state of FeH, predicted by theory to lie in this region, such an assignment is only tentative at this stage. Fragments of two other possible series of levels were seen in dispersed fluorescence from the e (6)Pi state. (C) 2001 Academic Press.