Ab initio investigation of high multiplicity Rþ—Rþ [sigma superscript + - sigma superscript +] 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+ [superscript 4 sigma superscript + - superscript 4 sigma superscript +] optical transition (dav=1.6 D) [(d subscript av = 1.6 D)] near 328 nm (T00 = 30460 cm-1) [(T subscript 00 = 30460 cm superscript -1)], analogous to the B 2Σ+ - X 2Σ+ [B superscript 2 sigma superscript + - X superscript 2 sigma superscript +] violet system, (dav=1.7 D) [(d subscript av = 2.2 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 (re=1.76 A, we = 365 cm-1) [(r subscript e = 1.76 angstrom, omega subscript e = 365 cm superscript -1)], and a 6Σ+–6Σ+ [superscript 6 sigma superscript + - superscript 6 sigma superscript +] transition, analogous to those for the doublet and quartet multiplicities, is predicted (dav=2.2 D) [(d subscript av = 2.2 D)]. Investigation of the isoelectronic BO, C-2 [C subscript 2 superscript -], and N+2 [N subscript 2 superscript +] molecules reveals that differences in 2s22px [2s superscript 2 2p superscript x] and 2s12px+1 [2s superscript 1 2p superscript x+1] atomic energies play the key role in determining the magnitude of the 5σ(2p)←4σ(2s)-derived [5 sigma (2p)← 4 sigma (2s)-derived] Σ+–Σ+ [sigma superscript + - sigma superscript +] transition energies for the different multiplicities. Furthermore, the strong stabilization of 2s22px [2s superscript 2 2p superscript x] character with respect to 2s12px+1 [2s superscript 1 2p superscript x+1] in BO and N+2 [N subscript 2 superscript +] leads to strongly bound lowest 6Σ+ [superscript 6 sigma superscript +] 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.