Molecular-orbital structure in neutron-rich Be and C isotopes

N. Itagaki; S. Okabe; K. Ikeda; I. Tanihata

doi:doi:10.1140/epja1339-07

2021 Impact factor 3.131

Hadrons and Nuclei

Eur. Phys. J. A 13, 43-47 (2002)

Molecular-orbital structure in neutron-rich Be and C isotopes

N. Itagaki¹, S. Okabe², K. Ikeda³ and I. Tanihata³

¹ Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
² Center for Information and Multimedia Studies, Hokkaido University, Japan
³ RIB Science Laboratory, RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan

(Received: 1 May 2001)

Abstract
The structure of Be and C isotopes are investigated based on the molecular-orbit (MO) model. The low-lying states are characterized by several configurations of valence neutrons, which are constructed as combinations of basic orbits. In ¹⁰Be, all of the observed positive-parity bands and the negative-parity bands are described within the model. The second 0⁺ state of ¹⁰Be has a large $\alpha$ - $\alpha$ cluster structure, and this is characterized by a $(1/2^+_\sigma)^2$ configuration. An enlargement of the $\alpha$ - $\alpha$ distance due to two-valence neutrons along the $\alpha$ - $\alpha$ axis makes their wave function smooth and reduces the kinetic energy drastically. Furthermore, the contribution of the spin-orbit interaction due to coupling between the S_z=0 and the S_z=1 configurations, is important. In the ground state of ¹²Be, the calculated energy exhibits similar characteristics, that the remarkable $\alpha$ clustering and the contribution of the spin-orbit interaction make the binding of the state with $(3/2^-_\pi)^2(1/2^+_\sigma)^2$ configuration properly stronger in comparison with the closed p-shell $(3/2^-_\pi)^2(1/2^-_\pi)^2$ configuration. This is related to the breaking of the N=8 (closed p-shell) neutron magic number. Also, the molecule-like structure of the C isotopes is investigated using a microscopic $\alpha$ + $\alpha$ + $\alpha$ + n+ n+ $\cdot \cdot \cdot$ model. The combination of the valence neutrons in the $\pi$ - and the $\sigma$ -orbit is promising to stabilize the linear-chain state against the breathing and bending modes, and it is found that the excited states of ¹⁶C are the most promising candidates for such structure.