CHANGE IN ADSORPTION BOND-LENGTH WITH COVERAGE FOR K AL(111)/

The local bond geometry of K adsorbed on Al(111) at low temperature ha s been studied by photoelectron diffraction (PED) as a function of K c overage. It is found that K atoms occupy on-top sites in the coverage range 0.05-0.4 monolayer and that the K-AI bond length increases by 0. 17 Angstrom over this coverage range. The reliability of this result i s supported by FED studies of the (root 3 x root 3)R30 degrees structu res formed by adsorption of one-third monolayer Na and K at 300 K, and K at 150 K, which give results in quantitative agreement with previou s structure determinations by SEXAFS and LEED.