Quenching of $1^{+}$ excitations in the double giant resonance

Abstract

The electromagnetic excitation of the two-phonon isovector giant dipole resonance in relativistic projectiles incident on heavy targets can be 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.

10 pages, 2 Postscript figures available upon request