The A 2Π–X 2Σ+ red and B 2Σ+–X 2Σ+ violet systems of the CN radical: Accurate multireference configuration interaction calculations of the radiative transition probabilities

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Abstract
Potential energy, dipole moment, and electronic transition moment functions have been calculated for the X 2Σ+, A2Π, and B 2Σ+ states of the CN radical using internally contracted CASSCF‐CI electronic wave functions (configuration interaction with complete active space self‐consistent‐field reference functions) and large basis sets. All molecular orbitals which can be formed from the atomic 2s and 2p orbitals were included in the active space. The effect of adding δ orbitals to the active space was found to be small. The largest calculations included up to 987 reference configurations and were equivalent to uncontracted MR‐CI calculations with 11.8 million configurations. These calculations are the most accurate that have been carried out to date in terms of the size of the basis set and the treatment of electron correlation effects. Using the theoretical transition moment functions and RKR potential energy functions, radiative lifetimes of the A 2Π and B 2Σ+ states have been derived. The lifetimes of the A state vary between 11.2 μs for v’=0 and 5.3 μs for v’=10. The lifetime of the B, v’=0 state is calculated to be 60.7 ns. These values are estimated to be accurate within 5% and are compared to previous experimental and theoretical data. The most recent experimentally measured lifetimes differ by 20% to 35% from the calculated values, while our results are in close agreement with other theoretical studies. Einstein coefficients of spontaneous emission and oscillator strengths for A–X, B–X, and B–A transitions are tabulated. Dipole moment functions have also been determined, and radiative transition probabilities between vibrational levels of the ground state are presented.