Generalized electric quadrupole branch linestrengths for the a1Δg-X3Σg- infrared atmospheric oxygen bands

The problem of branch linestrengths in a1Δ-3Σ electric quadrupole (E2) transition is addressed in generality. This is an intercombination transition, which derives its strength from spin-orbit perturbations. Considerations based on this premise lead to linestrength expressions involving two transition moments Q21 and Q22. The formulas are used to analyze the intensities of the S- and O-branch transitions in the a1Δg-X3Σg-, 0-0 band of the infrared atmospheric oxygen system measured recently (Gordon IE, Kassi S, Campargue A, Toon GC. J Quant Spectrosc Radiat Transfer 2010;111:1174-83) . The results show that although the major part of the E2 intensity component in the a-X, described by the moment Q22, is derived from the b1Σg+-a1Δg Noxon transition, the smaller moment Q21 arising from remote perturbers also contributes to the intensity up to a few percent.