Dipole Moment of HD

The HD molecule has a small permanent electric dipole moment as a consequence of the asymmetry of nuclear vibration. The complete molecular Hamiltonian contains a term which is not invariant to inversion of electronic coordinates through the molecular center. This gives rise to an axial asymmetry of electronic charge. Treating the term as a perturbation, it is found that the 1Σg+ electronic ground state is perturbed only by states of 1Σu+ symmetry. Assuming that only the lowest such state (the B state) contributes, an upper bound on the permanent moment is established. For the ground vibrational state this is computed to be 8.89×10−4 Debye unit, directed in the sense H+D−.