Optical properties of noble metal clusters as a function of the size:: Comparison between experiments and a semi-quantal theory

The optical properties of noble metal clusters are measured and compared in the size range 1.4-7 nm in diameter. The clusters of copper, silver and gold are produced by the same experimental technique, the deposition of preformed clusters in a transparent matrix. The size dependence is found to be different for the three metals. As the size decreases, the surface plasmon resonance is only slightly blue-shifted for silver, more strongly blue-shifted and damped for gold while this peak resonance vanishes for copper. We show that these results cannot be interpreted by a simple classical theory. Since ab initio calculations are not possible in this size range, we obtain a complete theoretical description of these optical properties through the same semi-quantal model for the three metals.