QRPA plus Phonon Coupling Model and the Photoabsorbtion Cross Section for $^{18,20,22}$O

We have calculated the electric dipole strength distributions in the unstable neutron rich oxygen isotopes $^{18,20,22}$O, in a model which include up to four quasi-particle-type configurations. The model is the extension, to include the effect of the pairing correlations, of a previous model very successful around closed shell nuclei, and it is based on the quasi-particle-phonon coupling. Low-lying dipole strength is found, which exhausts between 5 and 10% of the Thomas-Reiche-Kuhn (TRK) energy-weighted-sum-rule (EWSR) below 15 MeV excitation energy, in rather good agreement with recent experimental data. The role of the phonon coupling is shown to be crucial in order to obtain this result.