We have used the tunneling microscope to excite electron standing waves over clean and adsorbate-covered surfaces. For smooth, continuous overlayers we observe a resonance in the conductivity spectra that is absent on the clean surface, over film defects, and over film areas that are rough on a length scale of the electron’s de Broglie wavelength. By making reasonable assumptions of the adlayer thickness and interface scattering strength, the resonance can be modeled by a quantum-size effect in electron transmission through the adlayer.

• Received 7 June 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.2165