Theoretical Calculation of the Electronic States of C2+

Results of ab initio calculations are presented for a series of valence levels of C₂⁺ and for the two lowest states of ${\mathrm{C}}_2{\mathrm{(x}}^1{\Sigma }_g^{+}\mathrm{and}{\mathrm{X\prime }}^3{\Pi }_u)$ . Correlation energies of all the levels are estimated by a new scheme taking into account near‐degeneracy effects in the molecule and in the constituent ion pairs. Results for C₂ are in close agreement with experiment for equilibrium internuclear distances, vibration frequencies, term energies and dissociation energies. For C₂⁺ the calculations show the ground state to be ${}^4{\Sigma }_g^{-}$ with relatively low‐lying (∼ 0.6 eV) ${}^2{\Delta }_g$ and ${}^2{\Pi }_u$ levels. The electron‐impact measurement for the appearance potential of C₂⁺ is interpreted as $C_2{(}^3{\Pi }_u{\text{, υ\hspace{0.17em}=\hspace{0.17em}1)\hspace{0.17em}→\hspace{0.17em}C}}_2^{+}{(}^4{\Sigma }_g^{-}\text{, υ\hspace{0.17em}=\hspace{0.17em}0)}$ . The quartet transition ${}^4{\Sigma }_g^{-}{\mathrm{\hspace{0.17em}\to \hspace{0.17em}}}^4{\Sigma }_u^{-}$ is predicted to lie at 6000 Å. The major doublet transitions all lie in the infrared.