Effects of coupling between particles and surface vibrations on isoscalar response of nuclei

Abstract

The isoscalar quadrupole and octupole response of heavy spherical nuclei is studied in a semiclassical model that includes a coupling between the motion of nucleons and surface vibrations. By using a separable approximation for the residual interaction, an analytical expression has been obtained for the isoscalar response function of different multipolarity. A unified description of the low-and high-energy response is achieved within our semiclassical model. The inclusion of surface vibrations leads to the low-energy modes. The low-lying quadrupole mode is strongly affected by the surface tension but its frequency is still finite if, in the absence of Coulomb repulsion, the surface tension is set to zero. The frequency of the low-lying octupole mode, instead, goes to zero for vanishing surface tension, which would correspond to shape instability with respect to octupole deformation. Closed classical trajectories of triangular shape play an essential role in the damping of the low-lying octupole mode. Our octupole response displays also a novel resonance-like structure between the low-energy octupole resonance and the high-energy one (at around 13 MeV for a system with A=208 nucleons) that might have a toroidal character like the low-energy isoscalar dipole resonance.