A STRUCTURAL STUDY OF THE AL(111) (ROOT-3X-ROOT-3)R30-DEGREES-RB PHASE AT DIFFERENT TEMPERATURES

Normal-incidence standing x-ray wavefield (NISXW) measurements have be en made of the local adsorption site of Rb on Al(111) surfaces, partic ularly in an ordered (square-root 3 x square-root 3)R30-degrees phase, as a function of the sample temperature during adsorption or subseque nt annealing. The results confirm the atop-site occupation for low-tem perature (around 150 K) preparation, but show that room-temperature pr eparation leads to a structure having Rb atoms in surface substitution al sites. The overall structural situation is therefore essentially th e same as that found previously by low-energy electron diffraction LEE D for the Al(111)(square-root 3 x square-root 3)R30-degrees phases. Ho wever, experiments involving annealing of the low-temperature prepared surface to room temperature indicate that only a small part of the su rface easily transforms to the higher-temperature form, and indeed the re is evidence that even in room-temperature preparations some fractio n of the adsorbed atoms may remain in atop sites. The apparent conflic t of this result with that from recent photoemission core-level shift and LEED data is discussed.