Collective and single-particle aspects in the optical response of metal microclusters

The photoabsorption response of the spherical, neutral ${\mathrm{Na}}_8$,${\mathrm{Na}}_{20}$, and ${\mathrm{Na}}_{40}$ clusters is studied in the random-phase approximation built upon a jellium background. Special attention is given to ${\mathrm{Na}}_{40}$, which displays a particularly broad line shape. The theory provides an overall account of the observation in terms of the interplay between collective and individual electronic motion, and thus evidence for the importance of Landau damping in small metal clusters. The physics of this relaxation phenomenon as a quantum-size signature of the Coulomb interaction is analyzed through simple models.