The angular distribution for the reaction ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}(p, n){}_{}{}^{16}{}_{}{}^{}\mathrm{F}\left(0^{-}\right)$ has been measured at $E_p=35\mathrm{}$ MeV between 0° and 140° (lab), or momentum transfer 0.34-2.0 ${\mathrm{fm}}^{-1\mathrm{}}$. This is the first observation of an isovector $0^{+}$→$0^{-}$ transition, corresponding to a pure longitudinal spin response, at large momentum transfers. Distorted-wave Born-approximation calculations with the M3Y interaction give a reasonable account of the observed cross sections except at $1.4\lesssim q\lesssim 2.0$ ${\mathrm{fm}}^{-1\mathrm{}}$. This discrepancy might be due to the effects of the pion field in nuclei.

• Received 10 September 1982

DOI:https://doi.org/10.1103/PhysRevLett.49.1318