Microscopic studies on two-phonon giant resonances

A new class of giant resonances in nuclei, namely double-giant resonances, is discussed, They are giant resonances built on top of other giant resonan ces. Investigation on their properties, together with similar studies on lo w-lying two-phonon states, should give an answer on how far the harmonic pi cture of boson-type excitations holds in the finite fermion systems like at omic nuclei. The main attention in this review is paid to double-giant dipo le resonances (DGDR) which are observed in relativistic heavy ion collision s with very large cross sections. A great experimental and theoretical effo rt is underway to understand the reaction mechanism which leads to the exci tation of these states in nuclei, as well as the better microscopic underst anding of their properties. The Coulomb mechanism of the excitation of sing le- and double-giant resonances in heavy ion collision at different project ile energies is discussed in detail. A contribution of the nuclear excitati on to the total cross section of the reaction is also considered. The Coulo mb excitation of double resonances is described within both, the second-ord er perturbation theory approach and in coupled-channels calculation. The pr operties of single and double resonances are considered within the phenomen ologic harmonic vibrator model and microscopic quasiparticle-RPA approach. For the last we use the quasiparticle-phonon model (QPM) the basic ideas an d formalism of which are presented. The QPM predictions of the DGDR propert ies (energy centroids, widths, strength distributions, anharmonicities and excitation cross sections) are compared to predictions of harmonic vibrator model, results of other microscopic calculations and experimental data ava ilable. (C) 1999 Elsevier Science B.V. All rights reserved.