In the framework of ab initio pseudopotential calculations, a method is presented to compute an effective spin-orbit Hamiltonian within a nonorthogonal diabatic representation separating the ionic and covalent subspaces. The resulting Hund's case c adiabatic curves display long range structures. We predict that a small $\left(\approx 1{\mathrm{cm}}^{-1\mathrm{}}\right)$ fine-structure splitting of an ionic ${}_{}{}^3{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ curve is manifested in the spectrum of the excited N${\mathrm{a}}_2$ molecule slightly below the $(3p{}_{}{}^2{}_{}{}^{}P_{1/2}^{}{}_{}{}^{}+3p{}_{}{}^2{}_{}{}^{}P_{1/2}^{}{}_{}{}^{})$ asymptote as a series of well identified quadruplets. They should be observed in two-color photoassociation spectroscopy experiments using cold atoms.

• Received 28 November 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.2858