Coulomb excitation of double giant dipole resonances

We implement the Brink-Axel hypothesis for the excitation of the double giant dipole resonance (DGDR): The background states which couple to the one-phonon giant dipole resonance are themselves capable of dipole absorption. These states (and the ones which couple to the two-phonon resonance) are described in terms of the Gaussian Orthogonal Ensemble of random matrices. We use second-order time-dependent perturbation theory and calculate analytically the ensemble-averaged cross section for excitation of the DGDR. Numerical calculations illuminate the mechanism and are specifically performed for the reaction 208Pb+208Pb. We show that the contribution of the background states to the excitation of the DGDR is significant. At a projectile energy of 640 MeV/nucleon we find that the width of the DGDR, the energy-integrated cross section and the ratio of this quantity over the energy-integrated cross section for the single giant dipole resonance, all agree with experiment within experimental errors