Dynamic surface effects in the isoscalar dipole modes of heavy nuclei are
studied within a semiclassical model based on the solution of the Vlasov
kinetic equation for finite Fermi systems with a moving surface. In order to
clarify the role of dynamic surface effects we have considered an approximate
solution, which takes into account only the part of the variation of the
phase-space distribution function caused by the dynamic surface (the
dynamic-surface approximation). It is shown that the dynamic surface effects
have an essential influence on the features of the isoscalar dipole modes. The
isoscalar dipole strength function has a two-resonance structure already in the
dynamic-surface approximation, and the centroid energies of both the low-energy
resonance and the high-energy resonance are close to corresponding centroid
energies of exact strength function. Calculations of the velocity field in the
dynamic-surface approximation show the vortex character of the low-energy
isoscalar dipole resonance and the compression character of the high-energy
one.Comment: 15 pages, 4 figures, typos correc