Collective excitations in metallic clusters within the second random phase approximation

Abstract

The second random-phase approximation (SRPA) is a natural extension of the RPA. It essentially consists in enlarging the space of the elementary modes introduced to describe the collective states by adding 2 particle - 2 hole excitations to the 1 particle - 1 hole ones of the RPA. However SRPA calculations are really few and almost all are done by resorting to strong approximations. We apply the SRPA to the study of collective states in metallic clusters. All kinds of couplings among all 1 particle - 1 hole and 2 particle - 2 hole configurations are taken into account. The coupling of 1 particle - 1 hole configurations with the 2 particle - 2 hole ones strongly pushes down the multipole strength distribution, especially for the dipole case, with respect to RPA. The inclusion of the coupling of the 2 particle - 2 hole among themselves enhances this behaviour also for the collective states whose description within RPA is widely accepted as satisfactory. Some possible origins of these unpleasant results are discussed. In particular, the need for a better treatment of short range correlations is underlined. © 2009 IOP Publishing Ltd.