Ab initio study of the low-lying electronic states of the [formula omitted] anion.

Six low-lying states ( X 2 Σ g + , A 2 Π u , B 2 Σ u + , 4 Σ g + , 4 Π u and 4 Σ u + ) of C 2 − anion are studied by highly correlated ab initio calculations. The potential energy curves (PECs) are computed in the internuclear separation from 0.8 to 5.0 Å using the complete active space self-consistent field method, and then performing the internally contracted multireference configuration interaction with Davidson correction. Core–valence correlation and scalar relativistic effect are considered through employing the aug-cc-pcV5Z-dk basis set. The spin–orbit coupling of the A 2 Π u state is also calculated using Breit–Pauli Hamiltonian. Based on the PECs derived from the high-level ab initio calculations, the spectroscopic parameters are obtained by fitting the ro-vibrational levels that are acquired by solving the ro-vibrational Schrödinger equation. These spectroscopic parameters, especially for low-lying doublet states, are in good agreement with the previously calculated and experimental results. Finally, the electronic transition dipole moment matrix elements, Franck–Condon factors, radiative lifetimes and oscillator strengths of A 2 Π u – X 2 Σ g + and B 2 Σ u + - X 2 Σ g + are calculated and discussed as well. [ABSTRACT FROM AUTHOR]