Anomalous optical absorption in ultrathin Pb films

Ultrathin Pb films on Si(111)-(6x6)-Au deposited at 100 K are investigated by scanning tunneling microscopy, reflection high energy electron diffraction, and optical reflectance methods. From the reflectance data for light with a photon energy from 0.5 to 3 eV, the imaginary part of the Pb dielectric function epsilon(Pb) is determined. For films with a thickness of 12 ML, the Pb epsilon(Pb) vs photon energy resembles that expected for a bulk material. At smaller coverages the absorption spectrum shows anomalous, sharp optical structures. These structures are identified as strongly enchanced optical transitions characteristic of bulk materials. Simultaneously, a reduction of the free-electron Drude absorption is observed. This phenomenon is explained in terms of reduced electron screening. Our findings supply evidence of a bulklike electron band structure in ultrathin films and nanoparticles of Pb deposited on Si(111)-(6x6)Au.