The ground electronic state of B2

An accurate ab initio configuration interaction calculation has established that the ground electronic state of B2 is of 3Σ−g symmetry, and that the Douglas–Herzberg emission system near 3200 Å is due to transitions from the second 3Σ−u state to the X 3Σ−g state. The lowest 5Σ−u state, suggested by previous calculations as the ground state, is found to lie about 1300 cm−1 above the X 3Σ−g state. The assignment of the Douglas–Herzberg transition is based on the close agreement between calculated and observed molecular constants which, with observed values in parentheses, are Re(3Σ−g) =1.622(1.590) Å, ωe(3Σ−g) =988.5(1035.2) cm−1, Re(2 3Σ−u) =1.660(1.625) Å, ωe(2 3Σ−u) =884.5(929.3) cm−1, and Te(2 3Σ−u) =31 438(30 546.1) cm−1. Further support for this assignment is seen in the close agreement between calculated and observed isotope shifts and intensity distributions. An explanation is given for why the 1 3Σ−u −X 3Σ−g transition has not been observed.