Energy linkage between the singlet and triplet manifolds in LaH, and observation of new low-energy states.

Wavelength-resolved fluorescence spectra of jet-cooled LaH were obtained from D1, E1, and 0(+)((3)Σ(-)) states by exciting isolated rotational levels. The observation of a(3)Δ(1) and a(3)Δ(2) states at 1259.5(5) and 1646(1) cm(-1), respectively, established the missing energy link between the singlet and triplet manifolds. The low-energy b(3)Π(0,1) and B(1)Δ(2) states predicted earlier from ab initio studies were also observed for the first time. Vibrational constants ω(e) = 1418(2) cm(-1), ω(e)x(e) = 15.6(7) cm(-1) for the ground and ΔG(1∕2) = 1326.1(7) and 1312(1) cm(-1), respectively, for the a(3)Δ(1) and b(3)Π(1) states were also determined. Vibrational frequencies were found to be in excellent agreement with earlier ab initio values. However, ab initio term energies and spin-orbit separation of (3)Δ(2)-(3)Δ(1) and (3)Π(1)-(3)Π(0) were found to be in poor agreement with the present observations. Also, the (3)Π state that was predicted to be inverted is observed to be regular.