Isoscalar monopole excitations in 16O: α-cluster states at low energy and mean-field-type states at higher energy

Isoscalar monopole strength function in ¹⁶O up to E_x≃40 MeV is discussed. We found that the fine structures at the low-energy region up to E_x≃16 MeV in the experimental monopole strength function obtained by the ¹⁶O(α,α^') reaction can be rather satisfactorily reproduced within the framework of the 4α cluster model, while the gross three bump structures observed at the higher energy region (16≲E_x≲40 MeV) look likely to be approximately reconciled by the mean-field calculations such as random-phase approximation and quasirandom-phase approximation. In this paper, it is emphasized that 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 (E_x≲16 MeV), 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 (16≲E_x≲40 MeV). It is found that 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 α-clustering character. This dual nature of the ground state seems to be a common feature in light nuclei.