The vibrational modes of the surface have been measured using electron-energy loss spectroscopy. The measured mode energies are compared to dynamical models with parameters taken from effective medium theory. Strong Pd-Cu interplanar bonding gives rise to nearly degenerate Pd and Cu vibrations (95 cm - 1 ) at the X point, despite the large mass difference of the ions. Upon low temperature annealing of the surface, overlayer islands of pure Cu coalesce and order. These overlayer islands are characterized by a high-energy vibrational mode near 128 cm - 1 which grows in intensity upon annealing.