Collective dipole excitations in sodium clusters
  Some properties of small and medium sodium clusters are described within the RPA approach using a projected spherical single particle basis. The oscillator strengths calculated with a Schiff-like dipole transition operator and folded with Lorentzian functions are used to calculate the photoabsorbtion cross section spectra. The results are further employed to establish the dependence of the plasmon frequency on the number of cluster components. Static electric polarizabilities of the clusters excited in a RPA dipole state are also calculated.   Comparison of our results with the corresponding experimental data show an overall good agreement. 
A. A. Raduta, R. Budaca and Al. H. Raduta