Two-photon photoemission from silver nanoparticles on thin alumina films: Role of plasmon excitation

摘要

Two-photon photoemission (2PPE) from silver nanoparticles on a thin Al_2O_3 film on NiAl(110) has been investigated using femtosecond lasers. The 2PPE spectra show a feature similar to the surface state of Ag(111). The total 2PPE yield possesses a maximum at about 3.6 eV which corresponds to the surface plasmon resonance of the silver particles. For particle sizes of about 10 nm an enhancement by a factor 450 compared to Ag(111) is found. Up to this diameter the photoemission yield increases but levels off for larger sizes due to interactions between the particles. It is inferred that the 2PPE process is a direct transition via a virtual state, and that the enhanced 2PPE yield results from the enhancement of the electromagnetic field at the surface of nanoparticles.