On the breakdown of the Born-Oppenheimer approximation for a diatomic molecule

In this paper it is shown how the effects of the breakdown of the Born-Oppenheimer approximation can be allowed for in the determination of precise values for the bond length and the force constants of a diatomic molecule in a 1 Σ electronic ground state. It is also shown how other parameters, which characterize the extent of the breakdown of the Born-Oppenheimer approximation, can be determined from precise experimental data. The technique is applied to the infrared and microwave data that are available for the hydrogen halides, and for CO, and it is also shown how this theory can be used to interpret the discrepancy that exists between the theoretical ( ab initio ) and experimental vibration-rotation energies of H 2 and D 2 in their electronic ground state. The order of magnitude of the nuclear mass dependence of the bond lengths and force constants obtained by using the Born-Oppenheimer approximation is determined.