Coexisting electron emission mechanisms in small metal particles observed in fs-laser excited PEEM

Silver cluster films deposited on Si(1 1 1) were investigated by spectroscopic photoelectron microscopy using fs-laser excitation tuneable between hν = 1.45–1.65 eV and 2.9–3.3 eV. With increasing coverage the films grown as stepped wedges first exhibit clusters of few nanometers diameter with narrow size distributions that later agglomerate forming larger islands up to about 100 nm diameter. The cluster films have been characterized by SEM, AFM and HR-TEM. In the 3.1 eV range the small clusters emit more effectively and the dependence of electron yield on laser power follows a quadratic power law. Microspectroscopy reveals that the Fermi level onset is sharp(<150 meV width) and shifts by 2hν when the quantum energy is increased, thus confirming the predominance of two-photon-photoemission (2PPE). Under 1.6 eV excitation the situation is different: The power dependence is non-integer and the slope varies between 2.9 and 3.7 for different points on the sample. The Fermi edge appears smeared out and shifted by several hundred meV to lower final state energies. We attribute this deviation from pure 3PPE to thermally assisted nPPE of electrons from a transient “hot electron” gas in the nanoparticles.