He-diffraction studies of hydrogen adsorption on Pd(311)

The formation of ordered structures of hydrogen on Pd(311) at a substrate temperature of 110 K has been investigated with low-temperature He beams. We found, before the completion of a saturated c(1 x 1) phase, the formation of three low-coverage (2 x 1) phases (the notation refers to the centered rectangular unit cell of the substrate). The first (2 x 1)H structure is completed after an exposure of just 0.09 L, suggesting - with an initial sticking coefficient near unity - a coverage of 0.25 ML where hydrogen atoms adsorb along every second close-packed row. Upon additional H-2 exposure, this phase evolves into the (2 x 1)2H phase, which reaches its optimum order after 0.20 L and shows a glide symmetry plane along [2<(33)over bar>]. Progressive filling of the close-packed rows leads to the appearance of a (2 x 1)3H phase and finally to the formation of the c(1 x 1). Diffraction spectra for this phase show only slight changes upon H-2 exposure in the range 0.5-10 L. The existence of three low-coverage hydrogen overlayers for coverages <1 ML suggests an interaction mechanism between hydrogen atoms which is very different to the one present on Pd(110). Possible structural models for the reported phases are discussed.