A variety of clustering in $^{18}$O

We investigate excited states of $^{18}$O by using the antisymmetrized molecular dynamics. It is found that five different types of cluster states exist which we call ${}^{14}{\rm C}+\alpha$, higher-nodal ${}^{14}{\rm C}+\alpha$, two molecular states, and $4\alpha$ linear-chain. The calculated $\alpha$-decay widths are compared with the observed data. The higher-nodal ${}^{14}{\rm C}+\alpha$ cluster states reasonably agree with resonances reported by the recent experiments. We predict that the $\alpha$-particle emission is dominant for the ${}^{14}{\rm C}+\alpha$ cluster states while the molecular states prefer the $^{6}$He emission.