The Role Of Localized Surface-Plasmons In Photoemission From Silver Films - Direct And Indirect Transition Channels

We have examined electron photoemission spectra and scanning tunnelling microscopy images from rough (coldly deposited) and smooth (annealed) Ag films. Photoemission is induced by 1-photon and 2-photon processes under conditions of common total photon energy, polarization and light penetration depth. For smooth films we show that photoemission involves direct optical transitions and that the 2-photon excitation is a simultaneous absorption rather than a 2-step process. On rough films the 2-photon process is mediated by the excitation of localized surface plasmon modes, it is strongly enhanced, and it involves indirect, momentum non-conserving transitions. One-photon excitation on rough films is not induced by localized surface plasmons since the frequency of the light falls outside their resonance band. Low temperature scanning tunnelling microscope images of rough films directly show the roughness features on which localized surface plasmon activity is centred.