The evolution with beam energy of the prompt dipole radiation, related with entrance channel charge asymmetry effects, was studied in the fusion reactions: ³⁶Ar+⁹⁶Zr and ⁴⁰Ar+⁹²Zr at E_lab=16 and 15.1 MeV/u, respectively. Both reactions populate, through entrance channels having different charge asymmetries, the same compound nucleus at the same average excitation energy and with identical spin distribution. By studying the \gamma -ray energy spectra of the considered reactions, and by comparing the present result with previous ones obtained at lower energies, we deduce that the prompt dipole \gamma -ray emission presents a maximum value at 9 MeV/u and decreases toward lower and higher energies. Moreover, the centroid and the width of the preequilibrium dipole component were found to remain constant, within the errors, by increasing the beam energy.

PACS numbers: 24.30.Cz, 25.70.Gh