The static electric dipole polarizability of NaN clusters with even N has been calculated in a collective, axially averaged and a three-dimensional, finite-field approach for 2 ≤ N ≤ 20, including the ionic structure of the clusters. The validity of a collective model for the static response of small systems is demonstrated. Our density functional calculations verify the trends and fine structure seen in a recent experiment. A pseudopotential that reproduces the experimental bulk bond length and atomic energy levels leads to a substantial increase in the calculated polarizabilities, in better agreement with experiment. We relate remaining differences in the magnitude of the theoretical and experimental polarizabilities to the finite temperature present in the experiments. PACS. 36.40.-c Atomic and molecular clusters -31.15.E Density functional theory in atomic and molecular physics -33.15.Kr Properties of molecules, electric polarizability

Abstract:We discuss the optical response of small carbon chains from the linear to the non linear domain in the framework of Time Dependent Local Density Approximation. We show that even for moderate ionizations, corresponding to a moderately intense excitation, the optical response exhibits significant alteration with respect to the truly linear domain response. This reflects non trivial dynamical effects at the level of electrons.Keywords: optical response; carbon chains; linear and non-linear excitations.The physics of clusters in intense fields, in particular intense lasers fields, is currently motivating many experimental and theoretical studies [1][2][3][4][5][6][7][8][9][10]. Several experiments have been performed on rare gas clusters [3,5,6] as well as on metal clusters [2,4]. In both, rare gas and noble metal clusters, intense fields can easily ionize semi-core electrons which immediately take part into the response of the system. The details of the behavior of these stripped semi-core electrons is not yet fully understood. In particular, it is expected that these electrons remain for a sizable amount of time inside the clusters, and thus actively participate into the further excitation of the system. They can thus be heated up to keV energies and behave as a plasma which can undergo giant dipole oscillations, both effects leading to a complete and very energetic destruction of the system [2,6,11]. The theoretical description of such highly non-linear situations is still at its beginning [6,[8][9][10][11]. In particular, a proper understanding of the behavior of the (originally) rather deeply bound electrons requires the use of sophisticated microscopic methods. A good starting point is the fully fledged Time Dependent Local Density Approximation (TDLDA) as developed at various levels of refinements since a few years