Dominance of collective over proton transfer couplings in the fusion of ${}^{32}\mathrm{S}$ and ${}^{34}\mathrm{S}$ with ${}^{89}\mathrm{Y}$

Fusion excitation functions have been measured to high precision for the reactions ${}^{32}\mathrm{S}{+}^{89}\mathrm{Y}$ and ${}^{34}\mathrm{S}{+}^{89}\mathrm{Y}.$ The sub-barrier fusion cross sections for ${}^{32}\mathrm{S}{+}^{89}\mathrm{Y}$ are found to be more enhanced over single-barrier penetration calculations than those for ${}^{34}\mathrm{S}{+}^{89}\mathrm{Y},$ and the shapes of the two experimental barrier distributions are significantly different. The effects of proton transfer and double phonon excitations have been examined using exact coupled channels calculations. These suggest that the difference between the two systems lies mainly in the collectivity of the projectiles, whilst proton transfer has only a minor effect.