Infrared optical properties of ultrathin Fe films on MgO(001) beyond the percolation threshold

In ultrathin metallic films the scattering and the spatial distribution of charge are influenced by the interfaces with the vacuum and with the substrate, and by the resulting confinement of electrons. To investigate the dynamic conductivity of the free charge carriers, as the main contribution to the ir dielectric function, we used in situ transmission spectroscopy in the middle ir, which we performed during the evaporation of Fe on MgO(001) in ultrahigh vacuum. Up to a thickness of about 3 nm we find ir absorption and spectral dispersion clearly different from bulk iron behavior. For continuous films such findings indicate significant scattering of charge carriers at the surface and, at closer inspection of sets of spectra, a thickness dependent effective ir optical oscillator strength. The type of thickness dependence of surface scattering and of oscillator strength is correlated to the development of mesoscopic roughness during growth.