Multiple plasmons and anharmonic effects in small metallic clusters

Processes leading to possible anharmonic dipole-plasmon oscillations in alkali-metal clusters are investigated. The electronic excitation spectrum of sodium clusters including highly excited states with more than one plasmon is calculated with an approach based on the separation of center of mass and intrinsic motion of delocalized electrons. The spectrum is analyzed in terms of collective plasmon oscillations, intrinsic electron motion, and their interaction. The latter is of primary importance. It is responsible for the anharmonicity of giant dipole oscillations. The predicted nonlinear blueshift of the dipole resonance is comparable with its spreading width, and therefore ought to be observed in photoabsorption experiments.