ELECTRONS AT METAL-INSULATOR INTERFACES .1. THE EFFECT OF XE MONOLAYERS ON THE IMAGE POTENTIAL STATES OF AG(111)

To explore the properties of electrons at interfaces, image-potential states on metal surfaces with physisorbed overlayers are being examined. Using angle-resolved two-photon photoemission we have measured changes in the binding energy and dispersion of image states for a Xe monolayer on the Ag(111) surface. The binding energy of the n = 1 state shifts from its clean surface value of -0.76 +/- 0.02 eV to a value of -0.67 +/- 0.02 eV at monolayer coverage. Dispersion measurements show that the effective mass (m*) of the image potential state decreases from (1.35 +/- 0.10)m(e) to the free-electron value of (1.00 +/- 0.05)m(e). A model in which the only effect of the adsorbate is to change the work function of the metal approximates the binding energy shift, but fails to account for the effective mass. Finally, we suggest a model in which the unique electronic structure of the first adsorbate layer shifts the electron probability density outward from the surface. The dielectric screening expected within the layer thus becomes less significant. The coupling of the electron to the metal substrate is lessened and consequently the effective mass approaches that of a free electron.