Extensive multireference configuration interaction (MRD-CI) calculations carried out with a contracted 6s5p2dlf Gaussian basis set indicate that B2 + has a X2Σg +(σg 2σu 2σg) ground and a low-lying 12πu(σg 2σ u 2πu) state (Te = 0.25 eV). The computed dissociation energies De are 1.90 eV for X2Σg + and 1.65 eV for 12Πu. The adiabatic ionization potentials (IP) of B2 from X3Σg -(σg 2σu 2πu 2) into X2Σg +(πu 2 → σg, ∞) and into 12Πu(πu → ∞) are 8.99 and 9.24 eV, respectively; both results are expected to underestimate the true values by about 0.15 eV. The present data do not support recent experimental and ab initio results from Hanley, Whitten, and Anderson (J. Phys. Chem. 1988, 92, 5803). Those authors reported a 2Πu ground state, with a De of 0.8 ± 0.6 eV (0.7 eV) and an adiabatic IP (X3Σg - → 2Πu) of 10.3 eV (8.9 eV), with values in parentheses corresponding to their ab initio results. Since the photoelectron spectrum of B2 (not yet recorded to our knowledge) represents a valuable and alternative experimental technique for the spectroscopic characterization of low-lying states of B2 +, the IP's are investigated here for one-electron ionization processes having X3Σg -, 15Σu -, and 13Πu of B2 as lower state. The corresponding Franck-Condon factors are also reported.

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Journal Journal of Physical Chemistry
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Bruna, P.J. (Pablo J.), & Wright, J.S. (1990). Theoretical study of the ionization potentials of B2. Journal of Physical Chemistry, 94(5), 1774–1781.