Isoscalar Monopole Excitations and Cluster Structures in Light Nuclei

Isoscalar monopole excitation to cluster states in light nuclei is in general strong as to be comparable with the single particle strength and shares about 20 % of the sum rule value. The isoscalar monopole strength function in ¹⁶O is discussed up to ${E_x \lesssim 40 \, {\rm MeV}.}$ We found that (1) two different types of monopole excitations exist in ¹⁶O; one is the monopole excitation to cluster states which is dominant in the lower energy part, and the other is the monopole excitation of the mean-field type such as one-particle one-hole (1p1h) which is attributed mainly to the higher energy part, and (2) this character of the monopole excitations originates from the fact that the ground state of ¹⁶O with the dominant doubly closed shell structure has a duality of the mean-field-type as well as alpha-clustering character.