Resonant photoabsorption of metallic cluster in a strong laser field

A new approach to the resonance photoabsorption of metal clusters in strong laser fields is introduced. The method is based on the separation of the center of mass and the intrinsic motion of delocalized valence electrons. We calculate the excitation spectra of sodium clusters beyond the standard linear response theory, including highly excited states with more than one plasmon. We study the non-linear effects in photoabsorption of small sodium cluster in strong laser field. The degree of anharmonicity of dipole plasmon excitations is examined.We analyze the ionization of sodium clusters by intense laser pulse with photon energy close to the surface plasmon resonance. The proposed model describes the resonance channel of cluster ionization via excitation of double-plasmon collective electron state that subsequently decays by a single electron emission. The autoionization decay rate of two-plasmon excited state together with the energy and angular distributions of outgoing electrons is obtained for positively charged sodium clusters. Parameters of the laser pulse which are favorable for the resonant multi-plasmon ionization are analyzed.