Description of electric dipole excitations in the tin isotopes within   the quasiparticle time blocking approximation
  The quasiparticle time blocking approximation (QTBA) is applied to describe E1 excitations in the even-even tin isotopes. Within the model pairing correlations, two-quasiparticle (2q), and 2q*phonon configurations are included. Thus the QTBA is an extension of the quasiparticle random phase approximation to include quasiparticle-phonon coupling. Calculational formulas are presented in case of neutral excitations in the spherically symmetric system. The main equations are written in the coordinate representation that allows to take into account single-particle continuum completely. The E1 photoabsorption cross sections have been calculated in nuclei 116,120,124Sn. It has been obtained that the 2q*phonon configurations provide noticeable fragmentation of the giant dipole resonance resulting in appearance of significant spreading width. The results are compared with available experimental data. 
E. V. Litvinova and V. I. Tselyaev