A nomenclature for Λ-doublet levels in rotating linear molecules

It is proposed that the two Λ-doublet levels of linear molecules with nonzero electronic orbital angular momentum be labeled Λ(A′) and Λ(A″), e.g., Π(A′) and Π(A″) for Π states, etc., according to the following prescription: All series of levels in which the electronic wave function at high J is symmetric with reflection of the spatial coordinates of the electrons in the plane of rotation will be designated Λ(A′) for all values of J, and all those for which the electronic wave function is antisymmetric with respect to reflection will be denoted Λ(A″). It is emphasized that this notation is meant to supplement, and not replace, the accepted spectroscopic e/f labeling and the parity quantum number. The utility of the Λ(A′)/Λ(A″) notation is that it is of most relevance in the mechanistic interpretation of reactive or photodissociative processes involving open-shell molecules.