The ground electronic state of B2

Abstract
An accurate ab initio configuration interaction calculation has established that the ground electronic state of B2 is of 3Σg symmetry, and that the Douglas–Herzberg emission system near 3200 Å is due to transitions from the second 3Σu 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(2 3Σu) =1.660(1.625) Å, ωe(2 3Σu) =884.5(929.3) cm−1, and Te(2 3Σu) =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 3ΣuX 3Σg transition has not been observed.

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