  A nuclear structure model based on linear response theory (i.e., Random Phase
Approximation) and which includes pairing correlations and anharmonicities
(coupling with collective vibrations), has been implemented in such a way that
it can be applied on the same footing to magic as well as open-shell nuclei. As
applications, we have chosen to study the dipole excitations both in
well-known, stable isotopes like $^{208}$Pb and $^{120}$Sn as well as in the
neutron-rich, unstable $^{132}$Sn nucleus, by addressing in the latter case the
question about the nature of the low-lying strength. Our results suggest that
the model is reliable and predicts in all cases low-lying strength of non
collective nature.
