Summary

The research on the giant dipole resonances (GDR) built on excited states is relevant for the understanding of the properties of nuclear matter at finite temperature and of the evolution of the damping mechanisms of this collective vibration with temperature and angular momentum. Extensive experimental and theoretical work was done to address these topics. In particular, the nuclear shapes of hot nuclei are probed by the GDR and the nuclear deformations and their fluctuations affect the damping mechanisms. Generally, the intrinsic width is not affected by the temperature and the coupling to the shape ensemble characterizing the compound nucleus is responsible for the measured broadening of the GDR width. This broadening is due to both temperature and angular momentum and it was investigated at temperature up to approximately 3 MeV. Another quantity found to be independent of temperature is the Coulomb mixing of states with different isospin values. The knowledge of isospin mixing is very important also to evaluate beta decay probabilities. Some effort was also made to learn on the properties of the GDR in very heavy fissioning nuclei and in nuclei with extreme deformations (superdeformed, hyper-deformed, and Jacobi types) with the aim to shed light on how nuclei can obtain these exotic nuclear shapes.