Gold films grown on BaTiO3 substrates, illuminated by a monochromatic
beam in the visible range, were investigated using scanning tunneling
microscopy (STM). Irregular variations in the tip displacements versus
photon energy are observed at energies of the surface plasmon resonan
ces (2.0 +/- 0.1 and 2.7 +/- 0.1 eV) of the rough gold film.