The electromagnetic excitation process at beam energies around 500 MeV/u is
utilized to probe the dipole response of light neutron-rich nuclei. The se
condary beams are produced in fragmentation reactions and separated by the
GSI fragment separator FRS. By measuring the neutron decay after inelastic
scattering on a Pb target in a kinematically complete experiment, the diffe
rential electromagnetic excitation cross sections d sigma /dE* are derived.
The oxygen isotope chain is studied systematically ranging from A = 17 to
A = 22. Low-lying dipole strength is observed, exhausting about 10% of the
energy-weighted dipole sum rule for excitation energies below 15 MeV. Even
though this soft dipole excitation is present in all neutron-rich oxygen is
otopes investigated, the cluster sum rule associated with the dipole motion
of the valence nucleons against the O-16 core is only fully exhausted for
O-18. For the halo nuclei Be-11 and C-15,C-17 it is shown that detailed spe
ctroscopic information on the ground state single particle structure can be
deduced from the 'threshold' dipole strength.