Sign reversal of the spin-orbit constant for the C ³Π _u state of N₂

Ab initio calculations are performed at the multireference configuration-interaction level of theory on the diagonal spin-orbit functions for the lowest non-Rydberg states of Π3 u symmetry in molecular nitrogen. Spin-orbit constants deduced from the ab initio results confirm the recent suggestion, based on new experimental results, that the C Π3 u state of N2, long known to be regular in the region of its potential-energy curve minimum, becomes inverted at higher energies. By removing the effects of the crossing C′ Π3 u state, it is shown that Av for the C state changes sign from positive to negative near v=8, corresponding to a change in principal molecular-orbital configuration from (1 g) 2 (1 u) 2 (2 g) 2 (2 u) (3 g) 2 (1 π u) 4 (1 π g) to (1 g) 2 (1 u) 2 (2 g) 2 (2 u) 2 (3 g) (1 π u) 3 (1 π g) 2 at an internuclear distance near 1.4 Å.