Influence of steps on the interaction between adsorbed hydrogen atoms and a nickel surface

We have studied the effect of an extended array of defects on the two-dimensional phase behavior of adsorbed hydrogen on a Ni surface using helium atom scattering. Specifically, the interaction of hydrogen with the stepped Ni(977) surface was examined and compared with similar interactions with the flat Ni(111) surface. The phase behavior of hydrogen on Ni(977) is qualitatively the same as that of hydrogen on Ni(111); however, the temperature at which the order-disorder transition occurs is elevated. On the stepped surface, the ordered (2x2)-2H phase exists at a temperature 40 K higher than on the flat surface. This reversible phase transition is second order and is best fit with T-c=310 K and beta=0.12, indicative of two-dimensional Ising behavior. Stabilization of the ordered phase is attributed to pinning from the step edges. The cross section for diffuse elastic He scattering by adsorbed hydrogen and the temperature-dependent domain size of ordered hydrogen along the step edges are also discussed. (C) 1999 American Institute of Physics. [S0021-9606(99)01443-9].