Two-photon photoemission spectromicroscopy of noble metal clusters on surfaces studiedusing time-of-flight photoemission electron microscopy

Abstract

Time-of-flight two-photon photoemission spectromicroscopy was used to investigate Cusurface inhomogeneities and Ag cluster films deposited on a Si(111) substrate. Femtosecondlaser radiation with wavelength around 400 nm (photon energy 3.1 eV) was used forexcitation. The metal aggregates have linear dimensions in the range between 40 nm andseveral 100 nm. It is shown that the two-photon photoemission spectra of the Cu and Agnanoclusters reveal the same qualitative differences from the spectra of the correspondinghomogeneous and clean metal surfaces. In particular, they show an enhancedphotoemission yield (up to 70 times higher) and present a different overall shape,characterized by differences around the Fermi level onset and a steeper intensity increase atlower final state energies. These differences are discussed in terms of the excitation oflocalized surface plasmons in the clusters and a resulting modification of the near-zoneelectromagnetic field, which in turn influences the two-photon photoemission and itsdynamics. Moreover, it is shown that a positive unit charge (photohole) resides on theclusters during the timescale relevant for the two-photon photoemission process.