Spin forbidden transitions in the Ka = 0 subband of NO 2 in the λ = 592.5 nm region

In a high resolution laser excitation spectrum of NO ₂, lines were recorded which do not follow the selection rule ∆ N = ∆ J = ∆ F of "spin allowed" transitions. Line positions and intensities of these "spin forbidden" lines were investigated for all rotational lines up to N″ = 12 of the K_a = 0 subband around λ = 592.5 nm. While the observed line intensities of "spin allowed" transitions can be well described by the J-coupling scheme, neither the J- nor the G-coupling scheme sufficiently describes the "spin forbidden" transitions. The observations can be fitted satisfactorily by perturbation theory, in which the Fermi interaction in ²A₁ is treated as the perturber. This looks similar to a superposition of J and G scheme in the ²A₁ ground state.