Low-energy dipole excitations have been investigated theoretically in N=50,
several N=82 isotones and the Z=50 Sn isotopes. For this purpose a method
incorporating both HFB and multi-phonon QPM theory is applied. A concentration
of one-phonon dipole strength located below the neutron emission threshold has
been calculated in these nuclei. The analysis of the corresponding neutron and
proton dipole transition densities allows to assign a genuine pattern to the
low-energy excitations and making them distinct from the conventional GDR
modes. Analyzing also the QRPA wave functions of the states we can identify
these excitations as Pygmy Dipole Resonance (PDR) modes, recently studied also
in Sn and N=82 nuclei. The results for N=50 are exploratory for an experimental
project designed for the bremsstrahlung facility at the ELBE accelerator.Comment: Nuclear Physics in Astrophysics III Conference, 26 - 31 March 2007,
Forschungszentrum Dresden-Rossendorf, German
