Spin-orbit coupling and the interaction of molecules with the radiation field

The representation of spectroscopic transition amplitudes as a sum of multipolar interactions with the radiation field in the presence of spin-orbit coupling is investigated. It is found that electric dipole, magnetic dipole and electric quadrupole interactions can be invoked if matrix elements are taken between molecular states that are eigenstates of a Hamiltonian which includes spin-orbit coupling. A minor new term that contributes to singlet-triplet transition intensities is introduced.