Double-giant-dipole resonance in ^ Pb

Double-dipole excitations in ${}^{208}\mathrm{Pb}$ are analyzed within a microscopic model explicitly treating $2p2h$ excitations. Collective states built from such excitations are shown to appear at about twice the energy of the isovector giant dipole resonance, in agreement with experimental findings. The calculated cross section for Coulomb excitation at relativistic energies cannot explain simultaneously the measured single-dipole and double-dipole cross sections, however.