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 R_e,( ³Σ_g^-)=1.622(1.590) Å, ω_e(³Σ_g^-)=988.5(1035.2) cm^-1, R_e(2³Σ_u ^-)=1.660(1.625)Å, ω_e(2³Σ _u^-)=884.5(929.3) cm^-1, and T_e(2 ³Σ_u^-)=31438(30546.1) 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.