ALKALI-METAL ADSORPTION ON AL(111) AND AL(100)

Up to the beginning of this decade the physical properties of alkali-m etal adsorbates on metal substrates were seemingly soundly explained a nd drew no special attention aside from their known technological impo rtance related to catalysis and the dramatic lowering of the work func tion which they induce. Surprisingly, however, recent experimental and theoretical work has turned this situation around with reports of mos t unexpected behavior. We discuss density-functional theory calculatio ns for Na and K adsorbates on aluminum where new adsorbate geometries are predicted. The interplay and competition of the adsorbate-substrat e and the adsorbate-adsorbate interactions is shown to give rise to in teresting geometry changes (e.g. hollow, on-top, and substitutional si tes) and to a condensation phase transition of the (initially, mutuall y repelling) alkali adatoms into islands.