文摘
Time-of-flight photoemission electron microscopy was used to measure spatially resolved energy distribution curves of electrons emitted from Ag nanoparticle films with different mass thicknesses. Two-photon photoemission (2PPE) was induced by femtosecond laser pulse excitation with 3.1a0;eV photon energy and 200a0;fs pulse width. Regions of Ag nanoparticles with different average sizes and one region with a continuous 100a0;nm thick Ag film were deposited as a stepped wedge on a Si(1a0;1a0;1) substrate. Upon laser excitation the nanoparticle films exhibit a very high electron emission yield in the images, whereas the uncovered Si surface and the continuous Ag film are dark. The time-of-flight electron spectra obtained from the nanoparticle films are remarkably different from the spectra of the continuous film, well-known from literature. The nanoparticle spectra are up to a factor of 160 more intense than the spectrum of the continuous film. They reveal different widths, overall shape and a shift and broadening of the Fermi edge. The results are discussed in terms of Mie plasmon assisted two-photon photoemission of the nanoparticles.