In this contribution, we present measurements of the ultrafast dephasing time T2 of surface plasmon polariton excitation in gold nanoparticles by means of persistent spectral hole burning. T2 is an essential parameter that does not only reflect the role of different dephasing and deexcitation mechanisms but also allows one to determine the field enhancement factor that is of great importance for many applications of nanoparticles. In our experiments gold nanoparticles were first fabricated in ultrahigh vacuum on sapphire substrates by deposition of atoms, followed by diffusion and nucleation, i.e. Volmer-Weber growth. Subsequently, systematic measurements of T2 in the size range between r = 7 nm and 14 nm were carried out. The most essential among the numerous results is the observation of the influence of the reduced dimension on the dephasing time. While T2 = 14 fs has been measured for r = 12 nm which is, within the error bars, consistent with the damping
contained in the bulk dielectric function, the value of T2 shrinks to, for example, T2 = 11 fs for r = 7 nm. This reduction of T2 can be attributed to surface scattering of the electrons. Further experiments are in progress to confirm the predicted 1/r law for the variation of T2.