Cluster structure and isoscalar monopole excitation in light nuclei

Abstract

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. In the present paper, the isoscalar monopole strength function in 16O is discussed up to Ex ≲ 40 MeV as a typical example. We found that 1) two different types of monopole excitations exist in 16O; one is the monopole excitation of 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, 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, and 3) the monopole strength is much enhanced by the α-type ground state correlation.