Collective aspects of the dipole response in nuclei: an analysis from semi-classical and quantal approaches.

Within the framework of semiclassical and quantal approaches, we explore the dipole response properties in selected neutron-rich nuclei and in Sn isotopes. One of the goals of this analysis is to disentangle genuine collective aspects from single particles features. Focusing on the low- energy response, below the GDR region, we are able to identify one isovector-like mode and two isoscalar-like modes of considerable strength. In particular, owing to its mixed nature in neutron- rich nuclei, the lowest energy isoscalar-like mode also contributes to the isovector response and can be associated with the so-called Pygmy Dipole Resonance (PDR). We show that the isoscalar and isovector strengths of these low-energy modes evolve with nuclear global features, such as the density profile and neutron skin. In this sense, they also reflect important properties of the nuclear effective interaction. A thourough analysis of the associated transition densities turns out to be quite useful to better characterize the mixed nature of the different modes and their surface/volume components.