Covalent isomeric state in ¹²Be induced by two-neutron transfers

The α+⁸He low-energy reactions and the exotic structures of ¹²Be are studied using the generalized two-center cluster (α+α+4N) model. In the two-neutron transfer reactions, α+⁸He_g.s.→⁶He_g.s.+⁶He_g.s., a resonant peak with J^π=0⁺ appears around E~ 1.3 MeV above the ⁶He_g.s.+⁶He_g.s. threshold as the result of the formation of the covalent superdeformation, which has a hybrid structure of covalent and ionic configurations for the valence neutrons. The covalent superdeformation gives rise to an isomeric state with a sharp width of Γ~400 keV, which is smaller by about one order of magnitude than the typical width observed in molecular resonances above the Coulomb barrier. The energy-spin systematics for the two-neutron transfer reactions is investigated, and our calculation predicts a sequence of resonant structures in the range of 3–14 MeV in the center-of-mass energy with spins 0ℏ-8ℏ.