Experimental and theoretical evidence for a new metastable valence state of O₂

A new metastable state of O₂ has been observed following the charge exchange of fast beams of O₂⁺ with alkali metals. Coupling of the experimental results with ab initio calculations provides strong arguments for the identification of the metastable as the previously unknown 1¹Δ_u valence state which adiabatically dissociates to O(¹D) + O(¹D). The primary relaxation pathway is found to be a spin-orbit induced predissociation by the repulsive 2³Δ_u state for which an H_SO = 9 (±1) cm^-1 matrix element is determined. Due to accidentally small Franck-Condon factors the υ = 2 level of the 1¹Δ_u state is particularly long-lived and evidences extraordinarily large isotope effects with the lifetime for this level being 0.1, 1.8, and 0.5 μs for ³²O₂, ³⁴O₂, and ³⁶O₂ respectively. The other low vibrational levels of the 1¹Δ_u state are predicted to have lifetimes of ∼5-10 ns, suggesting that high-quality spectra of these states should be obtainable and that chemical reactions of this new state of O₂ are possible.