Observation of the second ³Π_u valence state of O₂

Rotational perturbations in the B ³Σ_u^- state of molecular oxygen are studied by measuring high-resolution vacuum ultraviolet laser photoabsorption cross sections of the (16,0)–(18,0) B ³Σ_u^-←X ³Σ_g^- Schumann-Runge bands of ¹⁶O₂. The observation of numerous extra lines in the spectrum, together with ab initio calculations of relevant potential-energy curves, enables the perturbing state to be assigned as the second valence state of ³Π_u symmetry. This weakly bound state, which we name C^′3Π_u, is also likely to be principally responsible for the well-known rapid increase in the magnitudes of the B-state triplet-splitting constants λ_v and γ_v as the dissociation limit is approached. © 1996 The American Physical Society.