The electric dipole response of exotic nuclei

The electric dipole (E1) response of exotic nuclei, with a focus on the soft E1 excitation of halo nuclei and the pygmy dipole resonance (PDR) of neutron-skin nuclei, is reviewed here from an experimental point of view. Since the discovery of halo nuclei in the late 1980s, the particular dipole response of these loosely bound nuclei located at the neutron-drip line resulting in an extraordinary enhancement of the electric dipole strength at low excitation energies (soft E1 excitation) has been one of the phenomena under intense investigation both in experiments and in nuclear theories. We discuss how such phenomena have been recognized as a unique feature of halo nuclei and how it is applied to probe the halo structure in a quantitative way. Another intriguing feature of the E1 response of neutron-rich nuclei is the PDR in neutron-skin nuclei, which is viewed as a dipole vibration of excess neutrons against a core nucleus and which appears in the low-energy part of the E1 strength function below the giant dipole resonance. The status of the experimental evidence for such a redistribution of dipole strength for neutron-rich nuclei compared to stable ones is reviewed. The relation of the dipole response to the symmetry energy in the equation of state of asymmetric nuclear matter and to the neutron-skin thickness is also briefly discussed. Finally, we outline some perspectives for future experimental investigations toward heavier and more neutron-rich systems. The experimental results reviewed here have been accumulated over the last two decades at fragmentation facilities worldwide at GANIL, GSI, MSU and RIKEN. (paper)