Pygmy Quadrupole Resonance in skin nuclei

The electric quadrupole response is investigated theoretically by HFB and QPM calculations along the Sn isotopic chain with special emphasis on excitations above the first collective state and below the particle threshold. Depending on the asymmetry, additional quadrupole strength clustering as a group of states similar to the known Pygmy Dipole Resonance is found. The spectral distributions, electric quadrupole response functions and transition densities of low-energy quadrupole states show special features being compatible with oscillations of neutron or proton skins against the nuclear core.