  The self-consistent Relativistic Quasiparticle Random Phase Approximation
(RQRPA) is extended by the quasiparticle-phonon coupling (QPC) model using the
Quasiparticle Time Blocking Approximation (QTBA). The method is formulated in
terms of the Bethe-Salpeter equation (BSE) in the two-quasiparticle space with
an energy-dependent two-quasiparticle residual interaction. This equation is
solved either in the basis of Dirac states forming the self-consistent solution
of the ground state or in the momentum representation. Pairing correlations are
treated within the Bardeen-Cooper-Schrieffer (BCS) model with a
monopole-monopole interaction. The same NL3 set of the coupling constants
generates the Dirac-Hartree-BCS single-quasiparticle spectrum, the static part
of the residual two-quasiparticle interaction and the quasiparticle-phonon
coupling amplitudes. A quantitative description of electric dipole excitations
in the chain of tin isotopes (Z=50) with the mass numbers A = 100, 106, 114,
116, 120, and 130 and in the chain of isotones with (N=50) 88-Sr, 90-Zr, 92-Mo
is performed within this framework.
  The RQRPA extended by the coupling to collective vibrations generates spectra
with a multitude of '2q+phonon' (two quasiparticles plus phonon) states
providing a noticeable fragmentation of the giant dipole resonance as well as
of the soft dipole mode (pygmy resonance) in the nuclei under investigation.
The results obtained for the photo absorption cross sections and for the
integrated contributions of the low-lying strength to the calculated dipole
spectra agree very well with the available experimental data.

Nuclear Theory