Spin-orbit and spin-rotation coupling in doublet states of diatomic molecules

The spin-rotation interaction and the centrifugal correction to the spin-orbit coupling make indistinguishable contributions to the energy levels of a diatomic molecule in a multiplet state with Λ ≠ 0. A previous method of separating these two contributions, based on the use of the vibrational dependence of the spin-orbit coupling constant, is unreliable. It is suggested here that a better procedure is one based on the isotope dependences of the spin-rotation coupling constant γ and the centrifugal correction to the spin-orbit coupling constant A_D. Both γ_e and A_De are shown to be inversely proportional to μ, the reduced mass of the molecule, but their contributions to the energy have different isotope effects. The method is used to determine values of γ_e and A_De for the X²Π state of HCl⁺, and the form of the spin-orbit coupling function A(r) in the vicinity of the equilibrium bond length is derived. The implications for RKR calculations are considered briefly.