Extended theory of finite Fermi systems: Application to the collective and noncollective E1 strength in {sup 208}Pb

The Extended Theory of Finite Fermi Systems is based on the conventional Landau-Migdal theory and includes the coupling to the low-lying phonons in a consistent way. The phonons give rise to a fragmentation of the single-particle strength and to a compression of the single-particle spectrum. Both effects are crucial for a quantitative understanding of nuclear structure properties. We demonstrate the effects on the electric dipole states in {sup 208}Pb (which possesses 50% more neutrons then protons) where we calculated the low-lying noncollective spectrum as well as the high-lying collective resonances. Below 8 MeV, where one expects the so-called isovector pygmy resonances, we also find a strong admixture of isoscalar strength that comes from the coupling to the high-lying isoscalar electric dipole resonance, which we obtain at about 22 MeV. The transition density of this resonance is very similar to the breathing mode, which we also calculated. We shall show that the extended theory is the correct approach for self-consistent calculations, where one starts with effective Lagrangians and effective Hamiltonians, respectively, if one wishes to describe simultaneously collective and noncollective properties of the nuclear spectrum. In all cases for which experimental data exist the agreement with the present theory results is good.