Strong Angular Momentum Effects in Near-Barrier Fusion Reactions

Cross sections and $\gamma$-ray multiplicities have been measured for neutron evaporation channels in the reactions ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ + ${}_{}{}^{144}{}_{}{}^{}\mathrm{Nd}$, ${}_{}{}^{37}{}_{}{}^{}\mathrm{Cl}$ + ${}_{}{}^{123}{}_{}{}^{}\mathrm{Sb}$, ${}_{}{}^{64}{}_{}{}^{}\mathrm{Ni}$ + ${}_{}{}^{96}{}_{}{}^{}\mathrm{Zr}$, and ${}_{}{}^{80}{}_{}{}^{}\mathrm{Se}$ + ${}_{}{}^{80}{}_{}{}^{}\mathrm{Se}$ leading to the compound system ${}_{}{}^{160}{}_{}{}^{}\mathrm{Er}$ at common excitation energies. In the near-barrier energy regime average angular momentum transfers depend dramatically on the asymmetry of the entrance channel. The results are interpreted in terms of the ground-state zero-point vibrations.