Contribution of density fluctuations to the damping of giant resonances

A generalized kinetic equation of Lennard-Balescu type is derived with retardation effects. By explicit consideration of the density-density fluctuations we give the contribution of fluctuations to the spreading width of giant resonances besides the collisional part. For small frequencies and long wavelengths, approximate formulas for the damping of giant resonances are derived. The density fluctuations couple the equation of state to the damping rate and give rise to an overall enhancement factor. This density and temperature dependent factor can reproduce the experimental values of the damping. Inside the spinodal region the factor turns out to be negative, indicating unstable modes. Explicit comparisons are made with the experimental values of giant monopole and dipole resonances. It is shown that the experimental widths can be described assuming finite nuclei since surface effects are important for the influence on density fluctuations. The recently reported saturation of the dipole width with increasing energy is explained as a transition between zero and first sound. Within this treatment we find an overcritical range of fluctuations, above which the nucleus is destroyed and no collective behavior is possible at all. copyright 1996 The American Physical Society