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 3Σg- state. The lowest 5Σu- state, suggested by previous calculations as the ground state, is found to lie about 1300 cm-1 above the X 3Σg- 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,( 3Σg-)=1.622(1.590) Å, ωe(3Σg-)=988.5(1035.2) cm-1, Re(23Σu-)=1.660(1.625)Å, ωe(23Σ u-)=884.5(929.3) cm-1, and Te(2 3Σu-)=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 3Σu--X 3Σg- transition has not been observed. © 1978 American Institute of Physics.