Curve Crossing of the B Σu−3 and Π u3 States of O2 and Its Relation to Predissociation in the Schumann—Runge Bands

Nonempirical quantum mechanical calculations including electron correlation have been carried out for the lowest Σu−3 and Π u3 states of O2. A relatively large basis set is used and 257 Σu−3 and 345 Π u3 symmetry-adapted configurations are included in the first-order wavefunctions. For the B Σu−3 state, the theoretical spectroscopic constants (with experimental values in parentheses) are Te 6.16 eV (6.17), De 0.76 eV (1.01), re 1.64Å(1.60), ω e 679 cm−1 (709), and Be 0.783 cm−1 (0.819). Neither state is well described by a single electron configuration, and the B Σu−3 state is seen to have a normal (non-Rydberg) electron distribution. The calculated potential curves indicate that the repulsive Π u3 curve crosses the inner limb of the B Σu−3 curve. Analogous calculations on the repulsive Π u1 state yield a crossing of the outer limb of the B Σu−3 Since previous interpretations of the predissociation of B Σu−3 have suggested that Π u3 crosses the outer limb, this predissociation is discussed in some detail. It is concluded that spin—orbit coupling is the principal interaction responsible for the predissociation, so that all four repulsive states that dissociate to ground state atoms are expected to predissociate B Σu−3 to roughly the same degree.