Ab initio investigation of high multiplicity Σ+-Σ+ optical transitions in the spectra of CN and isoelectronic species

Based on high-level ab initio calculations, we predict the existence of a strong 4Σ+-4Σ+ optical transition (dav=1.6D) near 328 nm (T00=30460cm-1), analogous to the B2Σ+-X2Σ+ violet system, (dav=1.7D) in the near-ultraviolet spectrum of CN. The lower state of the predicted transition is the lowest-lying state of quartet multiplicity and has been observed previously through its perturbations of the B state. The predicted transition will enable determination of the equilibrium properties of the metastable lowest quartet state of CN. The lowest energy metastable sextet state of CN is also calculated to be quasi-bound (re=1.76Å,ωe=365cm-1), and a 6Σ+-6Σ+ transition, analogous to those for the doublet and quartet multiplicities, is predicted (dav=2.2D). Investigation of the isoelectronic BO, C2-, and N2+ molecules reveals that differences in 2s22px and 2s12px+1 atomic energies play the key role in determining the magnitude of the 5σ(2p)←4σ(2s)-derived Σ+-Σ+ transition energies for the different multiplicities. Furthermore, the strong stabilization of 2s22px character with respect to 2s12px+1 in BO and N2+ leads to strongly bound lowest 6Σ+states with binding energies as high as 2.0 eV. We believe that these newly predicted sextet states could be identified through their perturbations of quartet states of the relevant molecules.