Acoustic Surface Plasmon Dispersion on Nanostructured Cu(111)

In the present paper, the acoustic surface plasmon (ASP) dispersion is investigated on nanostructured Cu(111) by angle-resolved high-resolution electron energy loss spectroscopy (HREELS). The surface is prepared by ion bombardment at different crystal temperatures and ion fluences, following the recipes given by J. Naumann et al. (Surf Sci 388:212-219, 1997). We demonstrate that, at room temperature, the ASP survives to the disordering introduced by sputtering. The slope of the dispersion curve is thereby slightly reduced with respect to the one measured on pristine Cu(111), in accord with the disorder induced up-shift of the Shockley surface state. When ion bombardment is performed at 393 K and the surface restructures at the nanoscale forming hexagonal holes, an additional sharp loss is observed at 0.87 eV for wave vectors around 0.15 Å -1, while the ASP loss gradually faints off with sputtering dose. Eventually, HREELS shows evidence only for the interband transition from the bottom of the Shockley surface state to the Fermi level.