SIMPLE MODE ON A HIGHLY EXCITED BACKGROUND - COLLECTIVE STRENGTH AND DAMPING IN THE CONTINUUM

Simple states, such as isobaric analog states or giant resonances, emb edded into continuum are typical for mesoscopic many-body quantum syst ems. Due to the coupling to compound states in the same energy range, a simple mode acquires a damping width (''internal'' dynamics). When s tudied experimentally with the aid of various reactions, such states r eveal enhanced cross sections in specific channels at corresponding re sonance energies (''external'' dynamics which include direct decay of a simple mode and decays of intrinsic compound states through their ow n channels). We consider the interplay between internal and external d ynamics using a general formalism of the effective non-Hermitian Hamil tonian and looking at the situation both from the ''inside'' (strength functions and spreading widths) and from the ''outside'' (S matrix, c ross sections, and delay times). The restoration of isospin purity and the disappearance of the collective strength of giant resonances at h igh excitation energy are discussed as important particular manifestat ions of this complex interplay.