HYDROGEN-INDUCED STRUCTURAL-CHANGES ON NIAL(110)

LEED I-V/analysis and surface X-ray scattering measurements have been used to determine the structural changes induced by the adsorption of atomic hydrogen on NiAl(110) at 130 K. The clean surface, ordered with 50at% Ni-50at%Al, relaxes away from bulk truncation to exhibit a larg e ripple. At 130 K the rippling (Al out, Ni in) is 0.19 Angstrom, as d etermined by LEED I-V The adsorption of atomic hydrogen reduces this r ippling by 16% at half of saturation coverage and at full saturation b y 44%. Saturation coverage was measured to be one hydrogen atom per (1 x1)surface unit cell(1 monolayer) using nuclear reaction analysis. Thi s observation contradicts first principles calculations that predict 1 monolayer of H removes the surface rippling.