Theory of multiphonon excitation in heavy-ion collisions.

We study the e ects of channel coupling in the excitation dynamics of giant resonances in relativistic heavy ions collisions. For this purpose, we use a semiclassical approximation to the Coupled-Channels problem and separate the Coulomb and the nuclear parts of the coupling into their main multipole components. In order to assess the importance of multi-step processes, we neglect the resonance widths and solve the set of coupled equations exactly. Finite widths are then considered. In this case, we handle the coupling of the ground state with the dominant Giant Dipole Resonance exactly and study the excitation of the remaining resonances within the Coupled-Channels Born Approximation. A comparison with recent experimental data is made.