Measurements of the dipole continuum response of nuclei provide information on collective as well as single-particle properties. The rapidly varying electromagnetic field of a high-Z target experienced by a fast moving projectile with several hundred MeV/u kinetic energy causes dipole transitions into the continuum up to excitation energies of the giant dipole resonance. The extraction of differential cross sections with respect to excitation energy, which are directly linked to the dipole strength functions, can be accomplished by an exclusive measurement of the decay. This method was applied in a series of experiments at GSI utilizing fast secondary beams produced via fragmentation aiming at an investigation how the dipole response of nuclei evolves as a function of increasing isospin. Results of the experimental programme, which has concentrated so far on light neutron-rich nuclei ranging from helium to oxygen isotopes, are discussed. Much in contrast to stable nuclei, low-lying dipole excitations well below the giant dipole resonance region have been observed as a general phenomenon for these neutron-proton asymmetric nuclei. A quantitative analysis of low-lying threshold strength for loosely bound nuclei indicates that the characteristics of the dipole strength is directly related to the ground-state single-particle structure of the valence nucleon in the projectile. Finally, a brief outlook on future perspectives is given.
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