Acoustic plasmon on the Au(111) surface.

We report an acoustic surface plasmon mode on the Au(111) surface, which disperses into the visible region, as measured by high resolution electron energy loss spectroscopy. The new mode is assigned to an acoustic surface plasmon arising from the Shockley-type surface state electrons and coexists with the conventional surface plasmon. This low energy collective excitation disperses linearly up to ∼2.2  eV, i.e., into the visible region. The divergence from theoretical prediction appears to emphasize the importance of band structure effects upon the dielectric function of the surface region.