Time-resolved two-photon photoemission spectroscopy of HOPG and Ag nanoparticles on HOPG

Time-resolved two-color two-photon photoemission experiments are used to in vestigate the electronic dynamics in highly oriented pyrolytic graphite (HO PG) and in Ag nanoparticles grown on HOPG. The multiphoton photoemission us ing 267 nm and 400 nm excitation is presented and discussed. For the first time the known unoccupied state on graphite 3.6 eV above the Fermi level ca n be identified as an image potential state. A lower limit of 60 fs is give n for the lifetime of this state. The two-color experiments reveal that the carrier relaxation deduced from time-resolved experiments is influenced by feeding of the intermediate states. First results of the strongly enhanced multiphoton photoemission from Ag nanoparticles on HOPG at 400 nm waveleng th are presented. This observation is explained by the excitation of the su rface plasmon and its subsequent decay.