Effects of the particle-particle channel on properties of low-lying vibrational states

Making use of the finite rank separable approach for the quasiparticle random phase approximation enables one to perform nuclear structure calculations in very large two-quasiparticle spaces. The approach is extended to take into account the residual particle-particle interaction. The calculations are performed by using Skyrme interactions in the particle-hole channel and density-dependent zero-range interactions in the particle-particle channel. To illustrate our approach, we study the properties of the lowest quadrupole states in the even-even nuclei ${}^{128}$Pd, ${}^{130}$Cd, ${}^{124-134}$Sn, ${}^{128-136}$Te and ${}^{136}$Xe.