Non-single-particle excitations in finite Fermi systems

In order to describe properly non−single−particle excitations in finite Fermi systems in terms of Green’s functions, it is necessary to build upon an approximation of the self−energy part which has the appropriate analytic properties. A distribution function Γ enables us to construct approximations of the self−energy part with suitable properties in a straightforward way. The formalism is applied to the case where the three lowest orders of the expansion of the self−energy part are given as a starting point, leading to a simple secular equation for the poles of this function. Excitations accompanying ionization in C3O2 are calculated as examples of non−single−particle excitations and are compared with experiment.