HE-SCATTERING STUDIES ON THE ADSORPTION OF OXYGEN ON RH(311)

The adsorption of oxygen on Rh(311) has been investigated by means of He scattering. The formation of ordered (2x1) and (1x3) structures was observed. (The notation refers to the centered rectangular unit cell of the substrate.) Ordered oxygen structures could only be prepared by annealing oxygen pre-exposed surfaces to at least 450 K. Both oxygen structures are stable up to 900 K. For the (2x1) phase evidence is presented that suggests that the oxygen atoms are adsorbed along every second close-packed row. Oxygen removal from the (1x3) phase by hydrogen at 430 K leads to a c(1x3) structure, which is metastable and reverts to the clean c(1x1) surface above 500 K. The presence of a pronounced rainbow pattern for the c(1x3) signals unambiguously that its structure is of the missing-row type. This suggests that oxygen induces a surface reconstruction in the (1x3) phase, whereby at least one out of three close-packed rows is missing and the oxygen atoms fill the resulting deep troughs.