Giant Resonances as a Probe of Collective Excitations in Atoms and Solids

Abstract

Collective effects involve coherent motion of a large number of particles under the influence of macroscopic mean fields. When it comes to electrons in a metal, or other types of plasmas, it is well known and accepted that there are collective elementary excitations, the so called plasmons, which govern the response of the metal to external perturbations. It is then natural to ask the question whether a finite electronic system also may be characterized in terms of collective behaviour. This is an old question, and the aswer is by now quite clear: It can indeed show collective behaviour. The “problem” is, however, that a finite system is ameanable to different types of treatments which in principle give the correct answers. This will naturally lead to a number of different views and interpretations of the same phenomenon, and even to different “schools” of thought.