Adsorption Probabilities of H2 and D2 on various Flat and Stepped Nickel Surface

From the measurement of the angular variation of the sticking coefficient and its change with beam energy one can conclude that adsorption of hydrogen on Ni(111) is an activated process whereas on Ni(110) no activation barrier exists. Adsorption on a stepped Ni(997) surface exhibits a pronounced minimum of the sticking coefficient at a gas temperature of 230 K. The activity of the steps for dissociation of H2 is confined to low gas temperatures. Detailed balancing is obeyed rigorously for adsorption and desorption on the Ni(111) and Ni(110) surface even at situations deviating from quasiequilibrium. On the stepped surface on the other hand establishing microscopic reversibility requires strict adherence to quasiequilibrium conditions. Permeation experiments lead to good agreement with low-temperature desorption on the Ni(111) surface but to complete disagreement with the data for the Ni(110) surface. As suggested by Comsa, permeating hydrogen in some cases seems to originate from a subsurface location rather than from an adsorbed state. Besides a small effect at low temperatures on the Ni(997) and Ni(110) surfaces no isotope effects for H2 and D2 are detected.