Electron dynamics in a heterogeneous system: thin Ag films on Si(1 0 0)

Photodesorption dynamics is strongly influenced by the electronic structure and electron dynamics of the substrate. Here we use time-resolved two-photon photoemission spectroscopy to investigate electron relaxation and transport in thin ultra-flat epitaxial Ag films on Si(1 0 0). Two-photon photoemission spectra obtained for excitation with 3.1 eV photons reveal that electron emission occurs preferentially from the Ag film. In contrast, the photoelectrons generated by two-photon excitation with 4.7 eV photons originate from the Si substrate. This conclusion is supported by intensity dependent spectral changes that can only be reconciled if both excitation steps occur in the Si substrate. In addition, this is corroborated by time-resolved experiments that reveal slow electron relaxation dynamics as it is expected for Si. Our results indicate that the excitation in the Si substrate and the transmission of electrons through the thin Ag film play an important role in electron dynamics in the heterogeneous system and can well be responsible for the film thickness and excitation wavelength dependence of photodesorption of NO2.