systems. Let us cite as an example the detailed studies of irradiation of molecules of biological interest coated by a finite and well known number of water molecules [@Liu06]. The study of the irradiation of finite molecular systems and clusters is thus not only of interest for basic science but also for a wide range of practical applications.

In all cases, the immediate electronic response of the irradiated system plays a key role as the doorway to all subsequent dynamical scenarios. A basic feature is here the optical response corresponding to electronic oscillations [@Mie08; @Kre93]. It delivers a first overview of the coupling between irradiation and matter in a large variety of dynamical situations, from gentle to strong perturbations [@Bra93; @Hee93; @Rei03a; @Saa06aR]. Optical response related to photo-absorption is the leading signal in the case of gentle perturbations. It has been explored in great detail for a large variety of electronic systems, from bulk down to atoms. For the case of stronger perturbations, further response channels, especially ionization, become highly relevant [@Cal00; @Rei03a; @Fen10]. Still, the optical response spectrum, which characterizes the structural coupling of the system to light, provides a highly valuable information on any ensuing response mechanism, especially 