TEMPERATURE-DEPENDENT LOCAL GEOMETRIES IN THE SYSTEM AL(100)-C(2X2)-NA - A SURFACE EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE STUDY

The adsorption of Na on Al(100) at 140 and 300 K has been studied by surface extended x-ray-absorption fine-structure experiments. An analysis of the measurements shows that the local geometry in the c (2 X 2)-Na phases formed at each temperature is completely different. At 140 K the Na atoms occupy fourfold hollow sites, whereas at 300 K a model is favored where the Na atoms lie beneath a reconstructed surface Al layer. Warming to 300 K causes an irreversible transformation of the low-temperature structure to the room-temperature structure.