  The dynamics of the surface plasmon in laser-driven metallic nanoparticles is
described by means of a master-equation formalism. Within the Markov
approximation, the dynamics is studied for different regimes ranging from weak
excitation in photoabsorption experiments to strong excitation in pump-probe
spectroscopy. It is shown that two collective levels are sufficient to describe
the dynamics of the surface plasmon. On this basis, we predict the appearance
of sidebands in the absorption spectrum of the probe laser field in pump-probe
experiments.
