Low-lying energy levels of the FeH molecule

High-resolution laser-excited fluorescence spectra of FeH at T similar to 350 K have been recorded and analysed to derive new information about some of the low-lying electronic states. In particular, transitions assigned to e(6)Pi-b(6)Pi have enabled the lowest rotational levels of upsilon = 0 in b(6)Pi(7/2), b(6)Pi(5/2), and, with less certainty, b(6)Pi(3/2), to be identified. The J = (7/2)(+/-) level of the upsilon = 0 level of the b(6)Pi(7/2) state lies 3902.93 cm(-1) above upsilon = 0, J = (7/2)(+/-) in X(4)Delta(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 X(4)Delta(7/3)), the Lambda-type doubling in the rotational levels of the new state is very small, no more than 0.04 cm-l at J = 11/2. This level appears to be upsilon = 0 of C(4)Phi(7/2), for which T-0(7/2)(+/-) is determined to be 3755.28 cm(-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 nb 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.