Dipole coupling channels for molecular anions

The problem of long-range dipole coupling of an electron to a rotating polar symmetric top molecule is investigated using a basis proposed by Engelking for near-threshold photodetachment of negative ions. Angular momentum coupling invariants are identified for the new basis and illustrate the fundamental difference of this approach from the usual fixed-nuclei approximation at higher energies. The dipole channels are found to have an intrinsic SO(4) Lie-group symmetry which simplifies the interpretation of computed channel structure. Application of second-order perturbation theory leads to estimates of the new channel invariant, which predict a leading-order threshold scaling law σkˣ, with $x<1$ for a molecular dipole moment μ<[3J(J+1)8K²]1/2. In cases where the total angular momentum of the system is equal to the angular momentum of the molecule (J), the dipole coupling becomes identical to that predicted for S-wave doubly excited states of two-electron atoms.