Intensities in Singlet-Triplet Bands of Diatomic Molecules

Intensity formulae are found for the rotational structure of bands arising from transitions between a singlet and a triplet state of a diatomic molecule. These intercombinations occur because the wave functions which diagonalize the orbit-spin interaction contain both singlet and triplet terms. In the transition ¹Σ-³Σ two cases arise, according as the two states have the same or opposite symmetry as regards reflection of the orbital motions in a plane containing the nuclei, a different set of branches appearing in either case. A comparison is made with measurements of intensities in the atmospheric absorption bands of oxygen; the agreement is satisfactory on the assumption that these bands are due to dipole transitions from the ³Σ^- ground state to a ³Σ^- state. Formulae are also found for the transitions ¹Σ-³Π, ¹Σ-³Δ when the triplet state comes under either of Hund's cases (a) or (b).