VIBRATIONAL-MODES OF THE CU(100)-C(2X2)-PD SURFACE

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. S trong Pd-Cu interplanar bonding gives rise to nearly degenerate Pd and Cu vibrations (95 cm(-1)) at the (X) over bar point, despite the larg e mass difference of the ions. Upon low-temperature annealing of the s urface, overlayer islands of pure Cu coalesce and order. These overlay er islands are characterized by a high-energy vibrational mode near 12 8 cm(-1) which grows in intensity upon annealing.