Effect of neutron excess on isovector dipole response of heavy nuclei

Isovector dipole excitations of spherical nuclei with neutron excess are studied within a semiclassical approach based on the Vlasov kinetic equation for finite systems with a moving surface. It is shown that the account of the surface degrees of freedom allows one to exclude spurious excitations related to the center-of-mass motion. It is found that the neutron excess leads to the appearance of a force in the energy range of a dipole pygmy-resonance