THE ADSORPTION OF H-2 ON FE(111) STUDIED BY THERMAL-ENERGY ATOM SCATTERING

The interaction of H-2 with Fe(111) has been studied by thermal energy atom scattering (TEAS). The specularly scattered He intensity as a fu nction of hydrogen coverage exhibits a concave drop in scattered He in tensity up to 30% coverage, followed by a plateau and another drop in intensity at 80% coverage. A model has been developed to account for t his data which assumes three adsorption sites for H on Fe(111), in ana logy with the three desorption peaks seen in temperature-programmed de sorption (TPD). The adsorption sites have not been definitely assigned , but are labeled ''deep-hollow,'' ''shallow-hollow,'' and ''on-top.'' Competitive adsorption into the deep-hollow and shallow-hollow sites is assumed to account for the initial concavity of the data. Adsorptio n into on-top sites only becomes significant at 80% coverage. Effectiv e cross sections and reflectivities for the three sites have been obta ined for theta(i) = 60-degrees and 40-degrees. When a H saturated surf ace is heated, the scattered He intensity decreases upon heating from 173 to 240 K, at which point desorption has already begun. This drop i n intensity is not completely explicable by a Debye-Waller attenuation . It is proposed to be due to a shift in population of the adsorbed H to more exposed, on-top sites, in accord with a Boltzmann distribution of adsorption sites. The scattered He intensity increases upon furthe r heating from 240 to 400 K, corresponding to the desorption of recomb ined H-2 from the surface. Comparison of the He/H/Fe(111) system is ma de with the He/H/Pt(111) and He/H/Fe(110) systems.