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Extended ab initio configuration interaction calculations of the potential curves of the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma g-,5\Sigma u-$, and $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma u-$ states of $B2$, have established that the ground state is of $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma g-$ symmetry. The Douglas-Herzberg emission $system1$ near 3200 $\backslash AA$ results from transitions between the second root of the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma u-$ symmetry and the ground $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma g-$ state. The lowest $\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}\Sigma u-$ state, suggested by previous theoretical $studies,2,3$ as the ground state, is found to lie approximately 1000 $cm-l$ above the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma g-$ state. The close agreement between calculated and experimental spectroscopic data identifies the states responsible for the Douglas-Herzberg transition. The calculated results, with the experimental values in parentheses, are:\begin{eqnarray*} T_{e} &=& 31438 cm^{-1} (30546.1)\ R_{e}(^{3}\Sigma_{g}^{-}) &=& 1.622 \AA (1.590)\ \omega_{e}(^{3}\Sigma_{g}^{-}) &=& 988.5 cm^{-1} (1035.2)\ R_{e}(^{3}\Sigma_{u}^{-}) &=& 1.660 \AA (1.625)\ \omega_{e}(^{3}\Sigma_{u}^{-}) &=& 884.5 cm^{-1} (929.3)\ \end{eqnarray*}