Two center molecular structures in light nuclear systems

In light neutron-excess systems, many kinds of molecular structures are discussed from the viewpoint of the clustering phenomena. In particular, much attention has been concentrated on Be isotopes, and their low-lying states are nicely described by the molecular orbit (MO) model based on the $^{\mathrm{8}}$Be $=\alpha +\alpha $ core. The neutron MO generated around $^{\mathrm{8}}$Be core, such as $\pi^{\mathrm{-}}$ and $\sigma ^{\mathrm{+\thinspace }}$associated with the covalent orbit of atomic molecules, have been shown to give a good description for the low-lying states of these isotopes. In addition, many resonant states, decaying into the fragments of $^{\mathrm{6,8}}$He, have been observed in recent experiments of Be isotopes. Furthermore, the experimental data of the highly excited states have been accumulated for other systems, $^{\mathrm{18}}$O, for instance. In this report, we will discuss the molecular structures, which are generated by various two center cores, such as $^{\mathrm{10,12}}$Be ($=\alpha +\alpha +$2,4N), $^{\mathrm{18}}$O ($=\alpha +^{\mathrm{12}}$C$+$2N) and $^{\mathrm{10,12}}$C ($=\alpha +\alpha +$2,4P). We employ the generalized two-center cluster model, which has been successful in the studies of $^{\mathrm{10,12}}$Be from bound region to continuum region. In particular, we focus on the variation of the molecular structure, which are generated by changing a combination of the cores and excess nucleons. The enhancement factors in reactions, which can identify the intrinsic structures, will also be discussed.