Dipole excitations below the neutron threshold in neutron rich Sn isotopes are studied theoretically in the Quasiparticle-Phonon Model with Hartree-Fock-Bogoliubov single particle input. Of special interest are the low-lying two-phonon 1 − states and the Pygmy Dipole Resonance (PDR). The evolution of low-energy dipole excitations with neutron excess is investigated over the Sn isotopic chain including the experimentally unknown regions close to 132 Sn. A dependence of the PDR strengths and centroid energies on the neutron skin thickness is found. Despite significant multiphonon contributions to mean energies and transition strengths, the PDR states retain their onephonon character. The fragmentation pattern is reduced with increasing neutron excess towards the N=82 shell closure which will be of advantage for future experimental work.PACS numbers: 21.60. Jz, 24.30.Cz, 27.60.+j A genuine feature of neutron-rich nuclei is the appearance of low-energy electric dipole strength, seen recently in high-precision photon scattering experiments already in stable nuclei with small [1, 2] and moderate [3] neutron excess. These so-called Pygmy Dipole Resonances (PDR) are observed as a clustering of states close to the neutron threshold which in presently accessible stable medium-and heavy-mass nuclei is at excitation energies E x ∼ 5.5−8 MeV. In a first attempt for experimental investigations on the dipole strength distribution in exotic nuclei with radioactive beams recently performed in MSU and GSI the neutron-rich (stable and unstable) oxygen isotopes are studied. A dipole strength is observed below [4] and above [5] the neutron threshold. The measurements of the latest in the energy region 5−8 MeV for 18,20 O 'are consistent with that for other nuclei that may exhibit the Pygmy Dipole Resonance'. Although carrying only a small fraction of the full dipole strength the PDR states are of particular interest because they are expected to reflect the motion of the neutron skin against the core of normal nuclear matter. In order to obtain more direct evidence for such a peculiar mode studies of low-energy dipole excitations over sufficiently long isotopic chains are necessary. Here, we present results of an exploratory theoretical investigation for the neutron-rich Sn isotopes in experimentally less or even unknown regions at neutron numbers N=70 − 82. As discussed in [3], the nature of the low-energy dipole strength differs significantly from the isovector Giant Dipole Resonance (GDR) mode where proton and neutron fluids as a whole move against each other. Moreover, the PDR mode has to be distinguished from the other known low-energy isoscalar dipole excitation, namely the two-phonon 1 − states resulting from the anharmonic interactions of the lowest 2 + and 3 − states in a nucleus. The anharmonicities are reflecting the intrinsic fermionic structure of the nuclear phonons thus deviating from ideal bosons. These competing effects are taken into account in an appropriate way by the Quasiparticle Phonon Model (QPM) [6]. Applic...

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A finite rank separable approximation for the particle-hole RPA calculations with Skyrme interactions is extended to take into account the pairing. As an illustration of the method energies and transition probabilities for the quadrupole and octupole excitations in some O, Ar, Sn and Pb isotopes are calculated. The values obtained within our approach are very close to those that were calculated within QRPA with the full Skyrme interaction. They are in reasonable agreement with experimental data.

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