Kinetic-theory approach to low-energy collective modes in nuclei

Two different solutions of the linearized Vlasov equation for finite system s, characterized by fixed- and moving-surface boundary conditions, are disc ussed in a unified perspective. A condition determining the eigenfrequencie s of collective nuclear oscillations, that can be obtained from the moving- surface solution, is studied for isoscalar vibrations of lowest multipolari ty. Analytic expressions for the friction and mass parameters related to th e low-energy surface excitations are derived and their value is compared to values given by other models. Both similarities and differences are found with respect to the other approaches, however the close agreement obtained in many cases with one of the other models suggests that, in spite of some important differences, the two approaches are substantially equivalent. The formalism based on the Vlasov equation is more transparent since it leads to analytical expressions that can be a basis for further improvement of th e model. (C) 1999 Elsevier Science B.V. All rights reserved.