Dynamics of single-particle and collective excitations in heavy nuclei

The propagation of single-particle and small-amplitude collective excitations in a heavy nucleus is considered. We calculate perturbatively corrections to the mean-field approximation induced by the coupling of one-particle and collective motion via the residual particle-hole interaction. Special attention is paid to the energy variation of the quasiparticle effective mass near Fermi energy. We conclude from the calculation that particles and holes excited in low multipolarity giant resonances have average effective masses of the order of 0.8 m rather than m. The mechanism for the decrease is provided by the enforced decoupling of the quasiparticles from surface oscillations due to the high frequency of the giant resonances. We also study the role of surface modes in the decay of giant resonances. Considerable reduction of the damping into 2p-2h states expected from the absorptive part of the optical potential is found. The correlated particle-hole pairs interact with each other by exchanging surface oscillations which adds a destructive interference term to the decay widths of giant resonances. The reduction depends on the multipolarity of the mode and is only large for low angular momenta.