Evidence for Giant Dipole Resonances Built on the Excited Prolate Band ofSi28

Measurements of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$+ ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ capture in which high-energy $\gamma$ rays were detected in coincidence with recoiling ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ nuclei are reported. The $\gamma$-ray spectra are dominated by strong transitions to the excited $K=0\mathrm{}$ prolate band of ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$. The excitation functions for the decays to the prolate states at 6.7 MeV ($0^{+}$) and 7.4 MeV ($2^{+}$) are consistent with giant dipole resonances. The width of these resonances is narrower than that of the oblate ground state, in surprising contrast to what is expected from energy and deformation considerations.