ENHANCED SURFACE VIBRATIONS AND RECONSTRUCTION OF THE AL(111) SURFACE-INDUCED BY RB ADSORPTION

The bond geometries of Rb atoms in two different (square-root 3 x squa re-root 3)R30-degrees structures, formed by adsorption on Al(111) at 1 00 and 300 K, have been determined by low energy electron diffraction. Adsorption at 300 K leads to occupation by Rb atoms of a quasisubstit utional, sixfold coordinate site formed by displacing every third Al a tom in the first layer of the substrate. Adsorption at 100 K leads to occupation of an on-top site on a rumpled Al layer in which the vibrat ional amplitudes of Al atoms are anomalously large. An irreversible, o rder-preserving phase transition from the 100 K to the 300 K structure occurs on annealing.