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

Two-photon photoemission (2PPE) from silver nanoparticles on a thin Al2O3 film on NiAl(1 1 0) has been investigated using femtosecond lasers. The 2PPE spectra show a feature similar to the surface state of Ag(1 1 1). 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(1 1 1) 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.