Coulomb excitation of double giant dipole resonances

We implement the Brink-Axel hypothesis for the excitation of the double gia nt dipole resonance (DGDR): the background states which couple to the one-p honon giant dipole resonance are themselves capable of dipole absorption. T hese states (and the ones which couple to the two-phonon resonance) are des cribed in terms of the gaussian orthogonal ensemble of random matrices. We use second-order time-dependent perturbation theory and calculate analytica lly the ensemble-averaged cross section for excitation of the DC;DR. Numeri cal calculations illuminate the mechanism and the dependence of the cross s ection on the various: parameters of the theory, and are specifically perfo rmed for the reaction Pb-208 + Pb-208 at projectile energy of 640 MeV/nucle on. We show that the contribution of the background states to the excitatio n of the DGDR is significant. We find that the width of the DC;DR, the ener gy-integrated cross section and the ratio of this quantity over the energy- integrated cross section for the single giant dipole resonance, all agree w ith experiment within experimental errors. We compare our approach with tha t of Carlson ct al, who have used a similar physical picture. (C) 2001 Else vier Science B.V. All rights reserved.