A series of calculations were performed which support the $\alpha$-particle core excitation model as a possible explanation of the structure of certain highly excited states seen in ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ and ${}_{}{}^{15}{}_{}{}^{}\mathrm{O}$. The features of interest are the apparent compression of the core states and the multiplet structure. The compression is semiquantitatively accounted for by an enlargement of the single particle well due to the presence of the $\alpha$ particle. The standard core excitation model fails to account for the multiplet structure. An operator resulting from the nucleon-nucleon spin-orbit interaction is found to account successfully for the multiplet structure. In these calculations, the model of the core is that of a $p_{\frac{3}{2}}$ hole coupled to the states of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$. Two different models of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ states are used and both give good results.

NUCLEAR STRUCTURE ${}_{}{}^{15}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$. Calculated levels, $J^{\pi }$. $\alpha$-nucleon spin-orbit interaction.

• Received 26 March 1974

DOI:https://doi.org/10.1103/PhysRevC.10.89