Fusion process studied with a preequilibrium giant dipole resonance in time-dependent Hartree-Fock theory

The equilibration of macroscopic degrees of freedom during the fusion of heavy nuclei, such as the charge and the shape, are studied in the time-dependent Hartree-Fock theory. The preequilibrium giant dipole resonance (GDR) is used to probe the fusion path. It is shown that such an isovector collective state is excited in N/Z asymmetric fusion and to a lesser extent in mass asymmetric systems. The characteristics of this GDR, such as its deformation, rotation, and vibration, are governed by the structure of the fused system in its preequilibrium phase. In particular, we show that a loweringq of the preequilibrium GDR energy is expected as compared to the statistical one. Revisiting experimental data, we extract evidence of this lowering for the first time. We also quantify the fusion-evaporation enhancement resulting from γ-ray emission from the preequilibrium GDR. This cooling mechanism along the fusion path may be suitable for synthesizing in the future superheavy elements using radioactive beams with strong N/Z asymmetries in the entrance channel.