The potential curves, transition energies (Te), and spectroscopic constants (Re, ωe) of several low-lying electronic states of B2 + and selected doubly excited states of B2 are given. The data have been obtained by using a multireference single- and double-excitation (MRD) configuration interaction (CI) approach and a triple-zeta plus polarization AO basis set. The B 2 + ground state, which is found here to be X 2Σg +, shows a rather shallow potential curve (Re = 4.015 bohr, ωe = 423 cm-1) when compared with that of X3Σg - of B2(Re = 3.00 bohr, ωe = 1051 cm -1, exptl.). The first excited state of B2 +, namely 1 2Πu, lies at Te = 0.30 eV. Moreover, double excitations relative to X 2Σg + are essential for the description of a large number of excited states, such as σuσg → πu 2 (1 4Σu -, 1 2Σu -, 1 2Δu, 1 2Σu +) and σu 2 → πu 2, (1 4Σ g -, 1 2Σg -, 1 2Δg, 1 2Σg +). Similarly, 2 2Πu arises from the triple excitation σu 2σg → πu 3. In the same order as given above, such multiple excitations lead to a significant gain in bond strength (i.e., shorter Re, larger ωe) as a result of the simultaneous depopulation and population of strongly antibonding and bonding MOs, respectively. For neutral B 2, similar features are observed for the excitation σu 2 → πu 2 relative to X 3Σg - (leading to 3 1Σ g +) and 1 3Πu (leading to 2 3Πu). As a result of the loss of the two πu electrons, the ground state dissociation energy (De) of B2 + (X 2Σg +), with a calculated value of 1.94 eV, is about 1.12 eV smaller than the experimental De of B2 (X 3Σ g -).

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Journal The Journal of Chemical Physics
Bruna, P.J. (Pablo J.), & Wright, J.S. (1989). Strongly bound multiply excited states of B2 + and B2. The Journal of Chemical Physics, 91(2), 1126–1136.