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

The formation of ordered structures of hydrogen on Pd(311) at a substrate temperature of 110K has been investigated with low-temperature He beams. We found, before the completion of a saturated c(1x1) phase, the formation of three low-coverage (2x1) phases (the notation refers to the centered rectangular unit cell of the substrate). The first (2x1)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 sub 2 exposure, this phase evolves into the (2x1)2H phase, which reaches its optimum order after 0.20 L and shows a glide symmetry plane along [233]. Progressive filling of the close-packed rows leads to the appearance of a (2x1)3H phase and finally to the formation of the c(1x1). Diffraction spectra for this phase show only slight changes upon H sub 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 than the one present on Pd(110). Possible structural models for the reported phases are discussed.