Molecular-orbital structure in light neutron-rich nuclei

The structures of Be isotopes and C isotopes are investigated using a microscopic α+α+n+n··· model and α+α+α+n+n···model, respectively, based on the molecular orbit (MO) model. The low-lying states of the Be isotopes are characterizes by several configurations of valence neutrons, which are constructed as combinations of three basic orbits. The model space employed is extended from that of conventional MO models, and the orbits are expressed as linear combinations of local Gaussians. Their positions are determined variationally. In 10Be, 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 10Be has a large α-α structure with a (1/2+)2 configuration. Increase of the α-α distance due to the existence of two valence neutrons along the α-α axis makes their wave function smooth and reduces the kinetic energy drastically. Here, the contribution of the spin-orbit interaction due to coupling between the Sz=0 and the Sz=1 configurations is important. The energy surface of 12Be is found to exhibit similar characteristics, with a significant α clustering and the contribution of the spin-orbit interaction causing the binding of the ground state with the (1/2+)2 configuration to become much stronger. As a result, the (3/2-)2(1/2+)2 configuration for the four valence neutrons almost 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 stability of the linear chain of 3α with respect to the breathing-like mode and the bending-like mode are investigated for various neutron configurations. It appears likely that a combination of the valence neutrons in the π- and the σ-orbit is to stabilize the system with respect to such modes, and it is found that the excited states of 16C with the (3/2π-)2(1/2σ-)2 configuration for four valence neutrons is one of the most probable of such structures. (author)