Identification of the many-line visible emission spectrum of BaO

The identity of the electronic transitions giving rise to the low-pressure (0.1–10 mTorr) ’’many-line’’ visible emission produced by the reactions of Ba(g)+N2O or O3 → BaO*+N2 or O2 has been elucidated by direct spectroscopic observations. Experiments that provided strong evidence regarding the origin of this complex, headless emission consisted of spectrally resolving BaO* (* denotes electronic excitation) visible chemiluminescence (4100–5500 Å) at pressures of 5–10 mTorr as the barium vapor–triatomic oxidant diffusion flame stoichiometry was switched from metal-rich to oxidant-rich. Under oxidant-rich conditions, excess N2O partially relaxed the barium monoxide emission to reveal that the nascent excited products of the reaction included both the A′ 1Π and A 1Σ+ electronic states of BaO. Identification of specific A′ → X 1Σ+ and A → X 1Σ+ vibrational transitions and analysis of their relative intensity distributions led to the conclusion that the complexity of the many-line spectrum is primarily the result of direct bimolecular reaction of barium vapor with nitrous oxide or ozone to produce two chemiluminescent electronic states, one (A 1Σ+ → X 1Σ+) intense and rotationally hot, and the other (A′ 1Π → X 1Σ+) weak in intensity and partially rotationally relaxed.