TRANSITION FROM 2D TO 3D COLLECTIVE RESPONSE - SURFACE SENSITIVITY

We present electronic excitation spectra calculated in the random phas e approximation (RPA) for bimetallic jellium films with widths ranging from zero to several Fermi wavelengths. We find that for q(parallel-t o) below roughly 0.1k(F), excitation frequencies converge to the surfa ce plasmon frequencies of the Drude model at strikingly different rate s as the slab thickens; and that this onset behavior is quite sensitiv e to the shape of surface potential barriers. At larger wavevectors, t he RPA response deviates from local behavior for all layer widths.