Exploring the role of low-lying intrinsic degrees of freedom and their impact on fusion cross-sections

The present work focuses on examining the low-lying intrinsic degrees of freedom and their impact on fusion dynamics. Fusion cross-sections were calculated using the coupled-channel code CCFULL for four specific reactions: $^{18}$O+$^{74}$Ge, $^{18}$O+$^{148}$Nd, $^{18}$O+$^{182}$W, and $^{18}$O+$^{186}$W, all conducted at energies below the Coulomb barrier across various energy levels. Vibrational and rotational features were studied concerning energy to distinguish their respective effects on fusion properties. The results indicate that the theoretical calculations for the nuclei $^{74}$Ge, $^{148}$Nd, $^{182}$W and $^{186}$W closely match the experimental data, particularly for the $2^+$ excited states. While slight discrepancies are observed for other excited states ($4^+$ and $6^+$), overall agreement remains significant. Additionally, the study reveals that hexadecapole deformation with different magnitudes have significant influences on the fusion cross-section. In cases where $\beta_4$ has a positive value, rotational levels beyond $6^+$ have minimal impact on the cross-section, resulting in a notable difference in the contribution of sequential channels. In contrast, for negative $\beta_4$ values, rotational energy levels up to the $2^+$ state substantially affect the fusion characteristics. Furthermore, the analysis extends to the estimation of the relative change ($\Delta\sigma_{fus}$) between the excited states and the ground state, both with and without considering coupling terms.