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

Abstract

Abstract. Time-resolved two-color two-photon photoemis-sion experiments are used to investigate the electronic dy-namics in highly oriented pyrolytic graphite (HOPG)andinAg nanoparticles grown on HOPG. The multiphoton photo-emission using 267nm and 400nm excitation is presentedand discussed. For the first time the known unoccupied stateon graphite 3 : 6eVabove the Fermi level can be identified asan image potential state. A lower limit of 60fs is given forthe lifetime of this state. The two-color experiments revealthat the carrier relaxation deduced from time-resolved experi-mentsis influencedby feedingof the intermediatestates. Firstresults of the strongly enhanced multiphoton photoemissionfrom Ag nanoparticles on HOPG at 400nm wavelength arepresented. This observation is explained by the excitation ofthe surface plasmon and its subsequent decay.PACS: 78.47.+p; 79.60.Jv; 79.60.-i; 82.65.JvGraphite and metallic nanoparticles grown on graphite arevery interesting systems for nonlinear optical studies usingtwo-photon photoemission spectroscopy (2PPES) and itstime-resolved counterpart. The electron dynamics of the puregraphite surface show distinct differences to that of a three-dimensional electron gas system like bulk metals, whichhave already been thoroughly investigated by time-resolved2PPES [1–4]. The interpretation of time-resolved 2PPES re-sults by Xu et al. [5] has recently initiated a controversyconcerning the hot carrier relaxation mechanisms in graph-ite. The authors explain the observed deviation from standardFermi liquid theory [6] by an additional plasmon mediatedrelaxation mechanism. However, this interpretation has beenquestioned [7] and the debate is still going on. Experimentalevidencelike the two-colortime-resolved2PPES experimentspresented here provides further substantial information.Another important issue is related to the study of theunoccupied states of graphite. With the development of in-verse photoemission spectroscopy (IPES) and target currentspectroscopy (TCS) the unoccupied states of graphite wereinvestigated [8–10]. Good agreement with theory was foundfor most of the observed states [11]. However, the assign-ment ofone state about3