OXYGEN-INDUCED ORDERING OF POTASSIUM IN A COADSORBATE PHASE - A SPA-LEED AND STM STUDY OF K-O/NI(100)

The potassium-oxygen/metal bonding configuration in the early stages of oxygen adsorption on potassium pre-covered Ni(100) surfaces has been studied using spot-profile-analysis low energy electron diffraction (SPA-LEED) and scanning tunnelling microscopy (STM). For an initial potassium coverage of 0.13 monolayers (ML), oxygen adsorption results in a (3 x 3) overlayer. This (3 x 3) structure has been imaged using STM and is interpreted as a primitive ordered potassium overlayer on top of the oxygen-covered substrate. Further LEED data are presented for potassium precoverages of 0.25 and 0.37 ML. The results for all three coverages suggest that oxygen atoms induce ordered potassium structures by providing a strongly corrugated potential. These findings are discussed in the context of current models of alkali-oxygen/metal bonding configurations.