Evolution of the prompt dipole {gamma}-ray emission with incident energy in fusion reactions

We investigated the prompt dipole {gamma}-ray emission, related with entrance channel charge asymmetry effects, in the {sup 32}S+{sup 100}Mo and {sup 36}S+{sup 96}Mo fusion reactions at E{sub lab}=196 MeV and 214.2 MeV, respectively, with the aim to probe its evolution with incident energy. These reactions populate, through entrance channels having different charge asymmetries, the {sup 132}Ce compound nucleus at an excitation energy of 117 MeV with identical spin distribution. Fusion events were selected by detecting high-energy {gamma} rays in coincidence with evaporation residues. The center-of-mass differential {gamma}-ray multiplicity spectra of the considered reactions were found to be identical within the experimental uncertainties in the whole energy range. This result, associated with that reported for the same reaction pair at higher beam energy where a larger giant dipole resonance yield was evidenced for the more charge asymmetric system, implies an increasing trend of the prompt dipole {gamma}-ray emission with incident energy. Calculations based on a collective bremsstrahlung analysis of the reaction dynamics are presented and compared with the experimental findings.