  The Photothermal Heterodyne Imaging method is used to study for the first
time the absorption spectra of individual gold nanoparticles with diameters
down to 5 nm. Intrinsic size effects wich result in a broadening of the Surface
Plasmon resonance are unambiguously observed. Dispersions in the peak energies
and homogeneous widths of the single particle resonances are revealed. The
experimental results are analysed within the frame of Mie theory.
