Pygmy dipole resonances in the tin region

The evolution of the low-energy electromagnetic dipole response with the neutron excess is investigated along the Sn isotopic chain within an approach incorporating Hartree-Fock-Bogoljubov (HFB) and multiphonon quasiparticle-phonon model (QPM) theory. General aspects of the relationship of nuclear skins and dipole sum rules are discussed. Neutron and proton transition densities serve to identify the pygmy dipole resonance (PDR) as a generic mode of excitation. The PDR is distinct from the GDR by its own characteristic pattern given by a mixture of isoscalar and isovector components. Results for the ¹⁰⁰Sn-¹³²Sn isotopes and the several $N=82$ isotones are presented. In the heavy Sn isotopes the PDR excitations are closely related to the thickness of the neutron skin. Approaching ¹⁰⁰Sn a gradual change from a neutron to a proton skin is found and the character of the PDR is changed correspondingly. A delicate balance between Coulomb and strong interaction effects is found. The fragmentation of the PDR strength in ¹²⁴Sn is investigated by multiphonon calculations. Recent measurements of the dipole response in 130,132Sn are well reproduced.