Chemisorption of Hydrogen on Tungsten (100)

The interaction of hydrogen with a single-crystal tungsten (100) surface has been studied, using LEED in conjunction with flash-desorption and surface-potential measurements. As hydrogen is adsorbed at room temperature a sequence of LEED patterns is observed, consisting of the formation of ``extra'' ½ ½ spots, splitting and eventual disappearance of these spots with simultaneous formation of ¼-order spots, and finally, at saturation, the disappearance of all extra diffraction features. The possibility that these LEED patterns are caused by diffraction in the adlayer rather than in a reconstructed tungsten lattice is discussed in some detail. The initial adsorption is found to occur with a sticking probability of 0.65 and is accompanied by dissociation of the hydrogen molecules. At saturation there are two hydrogen atoms per surface tungsten atom. The surface potential decreases approximately linearly with coverage and at saturation the total change is about −0.9 V. For coverages above 0.25 the adsorption proceeds with formation of a series of arrangements characterized by large superstructure units with ``out-of-step domains,'' the size of which depends on the density of adatoms. It appears that these structures cannot be explained by existing theories of adsorption and lattice statistics.