Dipole coupling channels for molecular anions

Abstract

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 $\ensuremath{\sigma}\ensuremath{\simeq}{k}^{x}$, with $x<1$ for a molecular dipole moment $\ensuremath{\mu}<{[\frac{3J(J+1)}{8{K}^{2}}]}^{\frac{1}{2}}$. 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.