Isospin Quintuplets inA=16Nuclei

Two experimentally observed $0^{+}$ and $2^{+}$ levels of ${\mathrm{O}}^{16}$ have been interpreted as analogs of ${\mathrm{C}}^{16}$ and ${\mathrm{N}}^{16}$ states, each being considered as belonging to isospin quintuplets, although the corresponding levels have not been observed in ${\mathrm{F}}^{16}$ and ${\mathrm{Ne}}^{16}$. Such $T=2\mathrm{}$ levels can be theoretically understood in terms of two-particle-two-hole excitations. Two perturbation methods are proposed. The first one is based on the knowledge of the $T=1\mathrm{}$ odd-parity-state wave functions. The second, the higher random-phase approximation with unperturbed ground state (Tamm-Dancoff), leads to a diagonalization in the space of operators defined by coupling one hole-hole pair with one particle-particle pair. The results of the two methods are comparable and in satisfactory agreement with experiment. In fact, a complete fit is reached in method II with a Serber-Yukawa interaction potential, while a Rosenfeld mixture also gives a rather good agreement.