Systematic of dipole excitations in the stable 134,135,136,137,138Ba isotopes

In this contribution the focus is on the systematic study of [2+ x3-]1-two-phonon excitations around the Z = 50 and N = 82 shell closures. The results for the Sn and Ba isotope chains will be shown in a general framework of all available data obtained by nuclear resonance fluorescence experiments. The photon scattering experiments referred to in this work were performed at the Bremsstrahlung facility of the 4.3 MV Stuttgart Dynamitron accelerator. NRF experiments are suitable for the determination of excitation energies, spins, lifetimes and transition strengths of low-spin states. The even-even isotopes show strong 1- excitations close to the sum energy of the 2+ and 3- phonons, interpreted as [2+ x3-]1-two phonon excitations. The behaviour of these [2+ x 3-]1-states in even-even nuclei can be explained by the so-called DCP effect. Furthermore, the coupling of a single nucleon to this collective excitation in the neighbouring odd-mass nuclei is investigated