Angular distributions of the energy spectra of photoneutrons emitted in the reaction 32S(γ, n0)31S were measured over the range of excitation energies from 18 to 29 MeV. Coefficients of Legendre polynomials fitted to the data gave no indication of photon absorption other than through electric dipole transitions, except at energies above 27 MeV. The measured value of the normalized Legendre coefficient a2 is − 1.0 ± 0.1 across most of the giant resonance region supports a simple shell model interpretation that the 2s12 → 2p32 single-particle transition (of channel spin 0) is the dominant mechanism for photoabsorption followed by neutron decay to the ground state of 31S. Comparison of the present results for the ground state channel with the total photoneutron cross section shows that up to an excitation energy of 18.5 MeV nearly all neutron-emitting transitions proceed to the ground state of 31S even though many channels for excited states are open. The ground state photoneutron channel exhausts about 6% of the TRK sum rule.

Differential cross sections have been measured for the reaction $^{18}\mathrm{O}$(\ensuremath{\gamma},${\mathrm{n}}_{0}$${)}^{17}$O over the region of excitation energy from 14 to 26 MeV. The angle-integrated cross section for the ground-state transition reveals that this channel accounts for less than 20% of the total photoneutron cross section in the structured pygmy resonance region (near 14 MeV) and is a small fraction (10--15 %) of the cross section in the region of the giant resonance (near 25 MeV). The values of angular distribution coefficients fitted to the data are consistent with a description of this reaction in which electric dipole excitations dominate the cross section in the pygmy resonance. Narrow regions exist near 15.0, 16.0, and 20.0 MeV where nonzero ${a}_{1}$ coefficients are observed indicating the absorption of non-E1 radiation. The measured cross section and ${a}_{2}$ coefficients are compared to a direct-semidirect calculation which gives reasonable agreement and suggests that f-wave neutron emission dominates the ground-state channel and that there is little justification for the introduction of E2 amplitudes other than a pure direct E2 term.

Photoneutron time-of-flight spectra from the $^{14}\mathrm{C}$(\ensuremath{\gamma},${\mathrm{n}}_{0}$${)}^{13}$C reaction were measured as functions of laboratory angle over the excitation energy region from 10 to 28 MeV. Angular distribution coefficients and differential cross sections were extracted as functions of excitation energy between 10 and 23 MeV. The angle-integrated ground-state cross section indicates that ground state transitions dominate the ${T}_{<}$ giant dipole resonance region below 13 MeV, but only contribute about 50% of the strength in the neutron channel in the rest of the giant dipole resonance region. The results support a mechanism of dominant E1 absorption in the energy region from 13 to 23 MeV where an average value of ${a}_{2}$=-0.5 indicates ${p}_{1/2}$\ensuremath{\rightarrow}${d}_{3/2}$ single-particle neutron transitions. Angular distribution information suggests that much of a prominent resonance at 11.3 MeV (with an integrated cross section of about 1.03 MeV mb) is due to an M1 transition from the ground state of $^{14}\mathrm{C}$. If this is the case, there is little fragmentation of the M1 strength in $^{12}\mathrm{C}$ brought about by the presence of valence neutrons. When combined with the observation of the lack of a pygmy E1 resonance below the giant dipole resonance region, these results suggest that a model of $^{14}\mathrm{C}$ as a $^{12}\mathrm{C}$ ``core'' with two valence, weakly coupled, neutrons is inappropriate. Below an excitation energy of about 19 MeV, there is reasonably good quantitative and qualitative agreement between the present data and the results of a recent shell model calculation.

Photoneutron energy spectra from the $^{16}\mathrm{O}(\ensuremath{\gamma},{\mathrm{n}}_{0})^{15}\mathrm{O}$ reaction were measured as functions of laboratory angle over a range of excitation energies from 30 to 35 MeV. Angular distribution Legendre-polynomial coefficients were extracted up to third order as functions of excitation energy. Nonzero values of the coefficients ${a}_{1} (+0.25\ifmmode\pm\else\textpm\fi{}0.02)$ and ${a}_{3} (\ensuremath{-}0.2\ifmmode\pm\else\textpm\fi{}0.02)$ were observed over the energy region explored, indicating interference between states of opposite parity. These values can be accounted for in a simple model incorporating electric quadrupole absorption strength decaying via the ground-state photoneutron channel. When combined with the previously-determined amplitude ratio for the $E1$ $p\ensuremath{\rightarrow}s$ and $p\ensuremath{\rightarrow}d$ single-particle transitions, the present results suggest that about 4% of the isovector energy-weighted sum rule is found in the ($\ensuremath{\gamma}$,${\mathrm{n}}_{0}$) channel in the energy range studied. The value of the ($\ensuremath{\gamma}$,${\mathrm{n}}_{0}$) cross section was found to vary from 1.5\ifmmode\pm\else\textpm\fi{}0.1 mb at ${E}_{x}=30$ MeV to 0.8\ifmmode\pm\else\textpm\fi{}0.1 mb at 35 MeV. The average magnitude of the $E2$ contribution to this cross section was estimated to be 0.05\ifmmode\pm\else\textpm\fi{}0.02 mb. This is in reasonable agreement with a recent continuum randomphase approximation shell model calculation, but is in disagreement with a previous measurement.