Knowledge Bank

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 molecule center. This gives rise to the axial asymmetry of electronic charge. Treating the term as a perturbation it is found that the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma g$ electronic ground state is perturbed only by states of $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma n$ symmetry. It is assumed that only the lowest such state (the B state) contributes. Averaging over the Morse eigenfunctions of the first three vibrational levels, the values $8.89\times 10-4,8.84\times 10-4,8.78\times 10-4$ Debye unit are obtained for the permanent moment. The dipole is directed along the internuclear axis with the proton end relatively positive.