Photoemission from multiply excited surface plasmons in Ag nanoparticles

Two-photon photoemission spectroscopy using femtosecond laser pulses is use d to investigate the excitation and decay mechanisms of the surface plasmon resonance in Ag nanoparticles grown on graphite. The resonant excitation o f this collective excitation leads to a two-orders-of-magnitude-enhanced tw o-photon photoemission yield from a graphite surface with Ag nanoparticles compared to the yield from pure graphite. From the shape of the photoemissi on spectra, the polarization dependence of the photoemission yield and the excitation probabilities for different excitation pathways we conclude that excitation with 400-nm femtosecond laser pulses leads to the coherent mult iple excitation of the surface plasmon in the Ag nanoparticles. This multip ly excited plasmon mode can decay via the coupling to a single-particle exc itation leading to the emission of an electron if its final state is locate d in the continuum. The surface plasmon in metallic nanoparticles is a mode l system to investigate collective excitations in multiphoton processes.