Quenching of 1+ excitations in the double giant resonance

The electromagnetic excitation of the two-phonon isovector giant dipole resonance in relativistic projectiles incident on heavy targets can proceed via several intermediate 1− one-phonon giant resonance states. In two-step electric dipole transitions the population of 0+, 1+, and 2+ two-phonon states are possible. We calculate the amplitude distribution of 1− excitations with an RPA formalism, and use it to calculate the electromagnetic excitation of two-phonon states in second order perturbation theory and coupled-channels. We show that a conspiracy between angular momentum coupling and the strength of the electromagnetic fields suppresses contributions of 1+ states to the total cross sections.