Evidence for isovector giant resonances at 2ħω via the(7Li,7Be) reactions onC12andSi28

We identify two electric isovector giant resonances at ${\mathit{E}}_{\mathit{x}}$=28 and 33 MeV in ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, and at ${\mathit{E}}_{\mathit{x}}$=27 and 31 MeV in ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$, distinguishing them from a continuum background in the ${(}^7$Li${,}^7$Be) reaction at ${\mathit{E}}_{\mathit{L}}$=26 MeV/nucleon and ${\mathrm{\theta }}_{\mathit{L}}$=0°. The shape of the underlying continuum is determined from a comparison of ΔS=0 and ΔS=1 energy spectra deduced with a ${}_{}{}^7{}_{}{}^{}\gamma$ coincidence method. The excitation energies of these resonances are consistent with those of the isovector giant quadrupole and monopole resonances previously observed by the electron scattering and the (${\mathrm{\pi }}^{\pm }$,${\mathrm{\pi }}^0$) reaction, respectively, though the widths presently observed are much narrower.