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.