The dipole strength distribution above the one-neutron separation energy was measured in the unstable ${}^{130}\mathrm{Sn}$ and the double-magic ${}^{132}\mathrm{Sn}$ isotopes. The results were deduced from Coulomb dissociation of secondary Sn beams with energies around $500\mathrm{MeV}/\mathrm{\text{nucleon}}$, produced by in-flight fission of a primary ${}^{238}\mathrm{U}$ beam. In addition to the giant dipole resonance, a resonancelike structure (“pygmy resonance”) is observed at a lower excitation energy around 10 MeV exhausting a few percent of the isovector $E1$ energy-weighted sum rule. The results are discussed in the context of a predicted new dipole mode of excess neutrons oscillating out of phase with the core nucleons.

• Received 29 April 2005

DOI:https://doi.org/10.1103/PhysRevLett.95.132501