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

Time-of-flight two-photon photoemission spectromicroscopy was used to investigate Cu surface inhomogeneities and Ag cluster films deposited on a Si(111) substrate. Femtosecond laser radiation with wavelength around 400 nm (photon energy 3.1 eV) was used for excitation. The metal aggregates have linear dimensions in the range between 40 nm and several 100 nm. It is shown that the two-photon photoemission spectra of the Cu and Ag nanoclusters reveal the same qualitative differences from the spectra of the corresponding homogeneous and clean metal surfaces. In particular, they show an enhanced photoemission 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 at lower final state energies. These differences are discussed in terms of the excitation of localized surface plasmons in the clusters and a resulting modification of the near-zone electromagnetic field, which in turn influences the two-photon photoemission and its dynamics. Moreover, it is shown that a positive unit charge (photohole) resides on the clusters during the timescale relevant for the two-photon photoemission process.