Monopole and dipole transitions of the cluster states of 18O

On the basis of an extended antisymmetrized molecular-dynamics calculation, we study the cluster structure of the ${0}^{+}$ and ${1}^{\\ensuremath{-}}$ states in $^{18}\\mathrm{O}$. We discuss that several different kinds of the cluster states appear in the excitation spectrum, and their monopole and dipole transitions are interesting fingerprints of unique cluster structure. We show that the monopole and dipole transitions are enhanced for the $^{14}\\mathrm{C}+\\phantom{\\rule{0.28em}{0ex}}\\ensuremath{\\alpha}$ cluster states, while they are hindered for the molecular-orbit state. We also point out that a ratio of the electric and isoscalar monopole transition strengths gives a good hint for the structure of the excited states.