Low-energy isoscalar dipole response of heavy nuclei within kinetic model.

The low-energy isoscalar dipole response of heavy spherical nuclei is studied by using a semiclassical model, based on the solution of the linearized Vlasov kinetic equation for finite Fermi systems. In this translation-invariant model the excitations of the center of mass motion are exactly separated from the internal ones. The low-energy dipole strength function displays three resonance structures in the energy region up to 15 MeV. Calculations of the velocity fields associated with the resonance structures at the centroid energies show a vortex (toroidal) character of two overlying resonances, while the origin of the lowest isoscalar dipole resonance structure is related to the dipole single-particle excitations. Its centroid energy is close to the minimum energy of the dipole single-particle spectrum. The main toroidal resonance gives a qualitative description of the low-energy isoscalar dipole resonance observed in heavy spherical nuclei.