Spectra of and Bonding in Diatomic Molecules

When we come to consider the electronic states of molecules we must realise that the spectra are now complicated by vibration or relative motion of the nuclei and rotation or orientation of the molecule in space. For- 

In a diatomic molecule the electrons move in a (usually strong) cylindrical electric field.The component of orbital angular momentum along the internuclear axis, described by a quantum number A, thus defines the electronic state. Where L is the orbital angular momentum vector of all the electrons, A takes the values A = 1=0, 1, 2. L. This is 

The coupling of the angular momenta in the molecule may proceed in a variety of ways known as Hund s coupling cases (a) through (e). These are summarized in Table VII. Mulliken s reviews - are recommended for a full account. 

In addition to the selection rule for the total angular momentum, as in atoms, and the rule of parity, we now have a conservation of symmetry rule for homonuclear diatomics, a rule for E states  

It is possible to correlate the molecular electronic states with the states of the separated atoms and independently, with the united atom having the same number of nucleons and electrons as the molecule. This correlation proceeds according to the Wigner-Witmer rules.and will be outlined in Appendix C. 

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