INTERFACIAL QUANTUM-WELL STATES OF XE AND KR ADSORBED ON AG(111)

The energies and dispersions of the image states and quantum well elec tronic states in layers of Xe and Kr on a Ag(lll) substrate were deter mined by angle-resolved two-photon photoemission (ARTPPE). For Xe, we measured binding energies of unoccupied electronic states for 1-9 laye rs and their parallel dispersion out to 4 layers. We measured the bind ing energies for a monolayer of Kr and dispersions for one and two lay ers. The n = 2 and n = 3 image states of the bare metal evolve into qu antum well states of the layer (states of the Xe conduction band discr etized by the boundary conditions of a 2-D slab) at higher Xe thicknes ses, where the n = 2,3 states exhibit both a perpendicular and paralle l dispersion similar to that of the bulk Xe conduction band. The n = 1 state appears to evolve with coverage as an image state screened by t he Xe layer, with appreciable electron density in the vacuum. A contin uum dielectric model (modified image state picture) reproduces the gro ss trends in the data, while an explicit quantum well analysis is used to extract the bulk Xe conduction band dispersion. A simple model whi ch takes into account the band structures of the substrate and the ove rlayer, as well as the image potential, gives good agreement with the binding energy data. The combination of high energy and momentum resol ution along both the surface parallel and surface normal yields very p recise measurements of the bulk Xe conduction band as well as informat ion about the behavior of conduction band electrons at interfaces. (C) 1996 American Institute of Physics.