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

We identify two electric isovector giant resonances at {ital E}{sub {ital x}}=28 and 33 MeV in {sup 12}C, and at {ital E}{sub {ital x}}=27 and 31 MeV in {sup 28}Si, distinguishing them from a continuum background in the ({sup 7}Li,{sup 7}Be) reaction at {ital E}{sub {ital L}}=26 MeV/nucleon and {theta}{sub {ital L}}=0{degree}. The shape of the underlying continuum is determined from a comparison of {Delta}{ital S}=0 and {Delta}{ital S}=1 energy spectra deduced with a {sup 7}Be-{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 ({pi}{sup {plus minus}},{pi}{sup 0}) reaction, respectively, though the widths presently observed are much narrower.