Diffusion and aggregation of Ag

The diffusion and aggregation of preformed Agn-clusters $(n=2{-}9)$ deposited onto a highly oriented pyrolytic graphite (HOPG) substrate is studied by two-photon-photoemission (2PPE). The sample is irradiated with ultrashort laser pulse pairs and the kinetic energy of the emitted photoelectrons is analyzed in a magnetic bottle type time-of-flight spectrometer. During annealing of the sample from 100 K up to room temperature, nanoparticles are formed on the surface by diffusion and aggregation of the silver clusters. A steep increase of the total photoelectron yield at a sample temperature of about 150 K is explained by the excitation of plasmons in the silver nanoparticles. From the kinetic energy distribution of the photoelectrons we deduce a strong variation of the work function of the sample during the formation of the nanoparticles, which is attributed to a quantum size effect