AN EXACT PHASE-SHIFT MODEL FOR IMAGE STATES ON METAL-SURFACES WITH ALKALI-METAL OVERLAYERS

Within the framework of multiple reflection theory, an exact solution is developed for image and surface states on a nearly-free-electron metal with a flat adlayer potential. Wave function matching conditions are used to determine the round-trip phase shift at a given energy, and a Bohr-like quantization condition is applied to determine bound state energies. Also, a model is developed for determining the effective mass of image state electrons in the presence of an adlayer. Binding energies obtained from two-photon photoemission experiments on Na/Cu(111) are in excellent agreement with the computed results.