Channel coupling effects in interactions of $$^{19}$$ 19 F with $$^{64,68}$$ 64 , 68 Zn at energies around the Coulomb barrier

The role of nucleon transfer channel coupling on the sub-barrier fusion excitation function has been elusive. Many studies have attributed a significant sub-barrier fusion cross-section enhancement over one-dimensional barrier penetration model (1d-BPM) calculation to nucleon transfer couplings. However, several systems exhibit no such enhancement besides having positive Q-value nucleon transfer channels.The objective is to delve into the role of coupling to various internal degrees of freedom on the fusion excitation functions in interactions of $$^{19}$$ F with $$^{64,68}$$ Zn. Fusion cross-section measurements are performed at energies $$\sim $$ 20 % above to $$\sim $$ 15 % below the Coulomb barrier using Heavy Ion Reaction Analyzer (HIRA) at Inter-University Accelerator Center (IUAC), New Delhi. Coupled-channel (CC) calculations, including coupling to vibrational states of $${^{64,68}}$$ Zn, rotational states of $${^{19}}$$ F, and nucleon transfer channels are performed. Cross-sections for various transfer channels are calculated to speculate their coupling effects on the fusion excitation functions. The results are compared on a reduced scale with neighboring systems involving $$^{19}$$ F and $$^{18}$$ O as the projectile. The sub-barrier fusion cross-sections of $${^{19}}$$ F $$+$$ $${^{64,68}}$$ Zn systems are enhanced by orders of magnitude compared to the corresponding 1d-BPM calculations. CC calculations performed with CCFULL, including coupling to collective excitations of reactants and pair transfer channel, failed to reproduce the experimental fusion cross-sections. GRAZING calculations show large one nucleon transfer cross-sections relative to other channels for the two systems. CCDEF calculation show that inclusion of coupling to one proton transfer channel is necessary to reproduce the experimental data for both systems.