Theoretical study of the spectroscopy of B2

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Abstract
The singlet, triplet, and quintet states of B2 below about 45 000 cm−1 have been studied at the multireference configuration‐interaction (MRCI) level in a [4s 3p 2d 1f] atomic natural orbital (ANO) Gaussian basis set. Calculations employing a [5s 4p 3d 2f 1g] ANO basis were also performed for the low‐lying states, X 3Σ−g, A 3Πu, a 5Σ−u, b1Δg, and c 1Σ+g, to assess the accuracy of the spectroscopic constants determined with the smaller basis and to position the singlet, triplet, and quintet manifolds as accurately as possible. Our best estimate of the Te value for the a 5Σ−u state is 1701 cm−1. A D0 value is computed for the X 3Σ−g state of B2 of 2.78 eV and estimate that D0 is 2.85±0.06 in the complete CI limit. This value is within the error bars of the rather uncertain JANAF value, but is larger than the value of 2.71 eV determined recently using the G1 approach. Einstein coefficients are presented for selected triplet–triplet transitions, and radiative lifetimes for the triplet states to help characterize the optical spectra. Finally the potential energy curves of B2 are compared to those determined in an analogous manner for Al2.