Singlet-triplet transitions diatomic molecules

Cj (Diatomic Carbon). The ground state is a singlet X L Dq(C—C) = 6.113 + 0.05 eV. Seven triplet and six singlet states have been found for Cj. The strongest and most easily excited system is the Swan bands, - in the 4300 to 67(X) A region. The emission lifetime has been measured by Fink and Welge (353) and is 0.2 + 0.05 /tsec. Electronic transition moments of various bands of the C2 molecule have been measured by Cooper and Nicholls (239). 

Fig. 7. Potential curves of ground and first excited singlet as well as first excited triplet states of a diatomic molecule. Some vibrational substates and their wave-functions are included. [Note In actuality the potential curves do not intersect. For a comprehensive discussion of this phenomena see the section on non-adiabatic transitions of reference (16). For our purposes we will assume that the potential curves do, in fact, intersect.]...

The general selection rules for electric-dipole allowed transitions in diatomic molecules are AA = 0, 1, AS = 0, g - u, and (for 2 states only) + <- + and — The selection rule on S is particularly limiting, because it implies that the relative energies of, for example, the singlet and triplet states in H2 cannot be... 

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