OXIDATION OF RUBIDIUM AT PLATINUM (111)

We have studied the adsorption of Rb and the co-adsorption of 02 and Rb at Pt(111) using LEED, AES, EELS, ARUPS excited by circularly polarised light (CDAD) and workfunction measurements. At T = 300 K different ordered structures occur as indicated by LEED. The pure Rb monolayer exhibits a (7 x 7) structure. Even at very low oxygen exposure ( < 0.05 L) this structure is changed to a (square-root 3 x square-root 3)R30-degrees structure with the same lattice spacing within 1 %. At high oxygen doses the adlayer becomes disordered. A (4 x 1) structure is observed after an additional heat treatment. Simultaneously, different stages of local order can be distinguished clearly by use of CDAD from the Rb 4p states. At T = 90 K disordered Rb adlayers were produced that show an almost vanishing CDAD. For high Rb coverage EELS and DELTAPHI measurements indicate that the suboxide Rb9O2 occurs during the early stage of oxidation.