Strongly bound doubly excited states of Be2

The triplet and singlet potential curves of Be2 generated by single and double excitations from 2σu into 3σg and (or) 1πu are studied with a multireference configuration interaction (MRD-CI) method. Relative to X1Σg +(2σg 22σu 2) with Re = 4.72 bohr and ωe = 258 cm-1 (calculated here), these antibonding MO → bonding MO excitations lead to average decreases in bond distance (in bohr) of 0.55 (2σu → 3σg), 0.88 (2σu → 1πu), 0.93 (2σu 2 → 3σg1πu), and 1.22 (2σu 2 → 1πu 2). The increase in vibrational frequencies ranges from 240 to 600 cm-1. The 3σg MO is found to be less bonding than 1πu, confirming predictions made by Bader et al. The experimental states A1Πu, and B1Σu + correspond to doubly excited 11Πu(2σu 2 → 3σg1πu) and singly excited 11Σu +(2σu → 3σg), respectively. The 13Σg - and 11Δg states, both 2σu 2 → 1πu 2, preserve their doubly excited structure up to dissociation. Within the Franck-Condon region, 13Πu changes from bound (2σu 2 → 3σg1πu) to repulsive (mixed 2σu → 1πg/2σg → 1πu), thereby creating the unusual situation of a strongly bound potential (short Re, high ωe) with an adiabatic dissociation energy near zero. The singlet counterpart 11Πu, however, behaves regularly as its doubly excited character is maintained up to large R(Be - Be)