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.
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