METASTABLE (1X2) MISSING-ROW RECONSTRUCTION OF PD(311)

The formation of ordered oxygen overlayers on Pd(311) as well as the s tructures obtained after their reduction in hydrogen have been studied by He scattering. Exposure of the clean surface to oxygen at 110 K an d subsequent annealing to 500 K leads to the formation of a well-order ed (2 x 4) pg structure. Oxygen removal from this phase by hydrogen at 320 K leads to the appearance of a (1 x 2)-reconstructed structure, w hich is metastable and reverts to the clean (1 x 1) surface at similar to 370 K. The presence of a pronounced rainbow pattern in the (1 x 2) in-plane spectra signals unambiguously that its structure is of the m issing-row type, suggesting that the reconstruction was induced by oxy gen in the (2 x 4)pg phase. He diffraction data from the metastable (1 x 2) structure also suggest that significant unreconstructed (1 x 1) patches coexist with the missing-row reconstruction. Possible structur al models for the (2 x 4) pg phase are also discussed. (C) 1997 Elsevi er Science B.V.