Excitation ofE1andM2resonances via (π−,γ) reactions onO16,18

The photon spectra from the reaction ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$(${\pi }^{-}$,$\gamma$)${}_{}{}^{16}{}_{}{}^{}\mathrm{N}$ and ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$(${\pi }^{-}$,$\gamma$)${}_{}{}^{18}{}_{}{}^{}\mathrm{N}$ were measured with a high-resolution pair spectrometer. Transitions to $2^{-}$ states having $T=1\mathrm{}$ in ${}_{}{}^{16}{}_{}{}^{}\mathrm{N}$ and $T=2\mathrm{}$ in ${}_{}{}^{18}{}_{}{}^{}\mathrm{N}$ appear to dominate. Excitation of $1^{-}$ states in the giant dipole resonance region, which are the analogs of the levels excited in photoabsorption and inelastic electron scattering on ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$, respectively, were also observed. The total and partial branching ratios for bound states and giant-resonance transitions are compared with shell-model predictions. Higher admixtures to the predominantly 1p-1h wave functions are found to be necessary to obtain agreement with the measured branching ratios. A new treatment of the continuum and resonance excitation, based on calculations of Ohtsubo et al., is compared with the data.