Λ Λ = 0 corresponds to the Σ Σ states. These states are non-degenerate, so that the states of a Σ Σ term can only be multiplied by a constant in a reflection through a plane containing the molecular axis. When Λ Λ = 0 , simultaneous eigenfunctions of H , L z and A y can be constructed. Since A y 2 = 1 , the eigenfunctions of A y have eigenvalues ± ± 1 . So to completely specify Σ Σ states of diatomic molecules, Σ Σ + states, which is left unchanged upon reflection in a plane containing the nuclei, needs to be distinguished from Σ Σ − − states, for which it changes sign in performing that operation.