The radiative and nonradiative decay rates of lissamine dye molecules, chemically attached to differently sized gold nanoparticles, are investigated by means of time-resolved fluorescence experiments. A pronounced fluorescence quenching is observed already for the smallest nanoparticles of $1\mathrm{n}\mathrm{m}$ radius. The quenching is caused not only by an increased nonradiative rate but, equally important, by a drastic decrease in the dye’s radiative rate. Assuming resonant energy transfer to be responsible for the nonradiative decay channel, we compare our experimental findings with theoretical results derived from the Gersten-Nitzan model.

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Using a near-field optical antenna effect, we measure the homogeneous line shape of the surface-plasmon resonance in single gold nanoparticles. The surface-plasmon dephasing times extracted from the near-field spectra of the individual particles vary around 8 fs. This mean value agrees with calculations based on Mie theory, which neglect surface effects. Deviations of individual particles from this value are interpreted as being due to variations in the local nanoenvironment. We also observe double-peaked line shapes caused by electromagnetic coupling between close-lying particles.