In the electric dipole approximation the transition amplitude for emission or absorption of radiation can be shown to be proportional to the vibronic matrix element of the component of the electric dipole operator D along the molecular axis. This is the permanent electric dipole moment. In homonuclear diatomic molecules, the permanent electric dipole moment vanishes and there is no pure rotation spectrum (but see N.B. below). Heteronuclear diatomic molecules possess a permanent electric dipole moment and exhibit spectra corresponding to rotational transitions, without change in the vibronic state. For Λ Λ = 0 , the selection rules for a rotational transition are: Δ Δ ℑ ℑ = ± ± 1 Δ Δ M ℑ ℑ = 0 , ± ± 1 . For Λ Λ ≠ ≠ 0 , the selection rules become: Δ Δ ℑ ℑ = 0 , ± ± 1 Δ Δ M ℑ ℑ = 0 , ± ± 1 .This is due to the fact that although the photon absorbed or emitted carries one unit of angular momentum, the nuclear rotation can change, with no change in ℑ ℑ , if the electronic angular momentum makes an equal and opposite change. Symmetry considerations require that the electric dipole moment of a diatomic molecule is directed along the internuclear line, and this leads to the additional selection rule Δ Δ Λ Λ = 0 .The pure rotational spectrum of a diatomic molecule consists of lines in the far infra-red or the microwave region, the frequencies of these lines given by: