Nonlinear Photoabsorption in Metallic Clusters

The optical response of simple metal clusters to strong laser fields should bear evidence of possible anharmonicities of dipole surface plasmon collective excitations. We present a theoretical method that allows one to calculate the excitation spectrum of sodium clusters beyond the linear response theory, including highly excited states with more than one plasmon. We make use of a coordinate transformation to separate center of mass (c.m.) and intrinsic motion of delocalized electrons. The coupling between c.m. motion and intrinsic excitations leads to some anharmonicity of the giant dipole oscillations. We calculate the photoabsorption spectrum of sodium clusters for increasing laser intensities and find sizeable anharmonic effects.

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