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

Abstract

Based on high-level ab initio calculations, we predict the existence of a strong ⁴Σ⁺–⁴Σ⁺ optical transition (d_av=1.6 D) near 328 nm (T₀₀=30460 cm^-1), analogous to the B ²Σ⁺–X ²Σ⁺ violet system, (d_av=1.7 D) 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 (r_e=1.76 Å, ω_e=365 cm^-1), and a ⁶Σ⁺–⁶Σ⁺ transition, analogous to those for the doublet and quartet multiplicities, is predicted (d_av=2.2 D). Investigation of the isoelectronic BO, C₂^-, and N₂⁺ molecules reveals that differences in 2s²2p^x and 2s¹2p^x+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 2s²2p^x character with respect to 2s¹2p^x+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.