Molecular-orbital structure in light neutron-rich nuclei

The structures of Be isotopes and C isotopes are investigated using a micro scopic alpha+alpha +n+n. . . model and alpha+alpha+alpha +n+n. . . model, r espectively, based on the molecular orbit (MO) model, The low-lying states of the Be isotopes are characterized by several configurations of valence n eutrons, which are constructed as combinations of three basic orbits. The m odel space employed is extended from that of conventional MO models, and th e orbits are expressed as linear combinations of local Gaussians. Their pos itions are determined variationally. In Be-10, all of the observed positive -parity bands and negative-parity bands are described within the model. It is found that the second 0(+) state of Be-10 has a large ce-a structure wit h a (1/2(+))(2) configuration. Increase of the a-a distance due to the exis tence of two valence neutrons along the a-a axis makes their wave function smooth and reduces the kinetic energy drastically. Here, the contribution o f the spin-orbit interaction due to coupling between the S-z = 0 and the S- z = 1 configurations is important. The energy surface of Be-12 is found to exhibit similar characteristics, with a significant alpha clustering and th e contribution of the spin-orbit interaction causing the binding of the gro und state with the (1/2(+))(2) configuration to become much stronger. As a result, the (3/2(-))2 (1/2+)' configuration for the four valence neutrons a lmost degenerate with the (3/2-)2 (1/2(+))2 configuration. This is related to the breaking of the N = 8 magic number. For the C isotopes, the stabilit y of the linear chain of 3 alpha with respect to the breathing-like mode an d the bending-like mode are investigated for various neutron configurations . It appears likely that a combination of the valence neutrons in the pi- a nd the sigma -orbit is to stabilize the system with respect to such modes, and it is found that the excited states of C-16 with the (3/2(pi)(-))(2) (1 /2(sigma)(-))(2) configuration for four valence neutrons is one of the most probable of such structures.