High resolution absorption spectrum of CO₂between 1750 and 2000 Å. 2. Rotational analysis of two parallel-type bands assigned to the lowest electronic transition 1³B₂←¹

Abstract

The absorption spectrum of CO₂ gas between 175 and 200 nm was photographed at high resolution some years ago. This very weak spectral region proved to be extremely rich in bands showing rotational fine structure. In Part 1 [C. Cossart-Magos, F. Launay, J. E. Parkin, Mol. Phys., 75, 835 (1992), nine perpendicular-type bands were assigned to the lowest singlet–singlet transition, 1¹A₂ ←

, a forbidden transition rendered vibronically allowed by excitation of one quantum of the asymmetric stretching ν′₃ (b₂) vibration. Here, the parallel-type bands observed at 185.7 and 175.6 nm are assigned to the lowest triplet–singlet transition, 1³B₂ ←

excited with different quanta of the bending ν′₂ (a₁) vibration. The assignment and the rotational and spin constant values obtained are discussed in relation to previous experimental data and ab initio calculation results on the lowest excited states of CO₂. The actual role of the 1³B₂ state in CO₂ photodissociation, O(³P)+CO(X¹Σ⁺) recombination, and O(¹D) emission quenching by CO(X) molecules is reviewed.