Energy Levels, Selection Rules, and Line Intensities for Molecular Beam Electric Resonance Experiments with Diatomic Molecules

In the molecular beam electric resonance method the energy levels in an electric field of a diatomic molecule which is in the ground electronic state, and a low vibrational and rotational state are studied. Transitions are produced between states with different space quantization of the molecule relative to the electric field and with different couplings of the angular momenta of the nuclei and of the molecular rotation. Thus the interaction of the molecule with the field and the internal molecular interactions (e.g., nuclear electrical quadrupole interactions and nuclear spin—molecular orbit interactions) are measured. In this paper we give the stationary state energy values and eigenfunctions for "very weak," "weak," and "strong" field conditions for a diatomic molecule in which one nucleus has a spin of ½. Selection rules for the transitions are developed and some considerations on line intensities are discussed. Finally, there is presented the theory of double quantum transitions, which are predicted to occur at one-half the frequency given by the Bohr condition, $\Delta W=h\nu$, provided the radiofrequency field is sufficiently intense.