ADSORPTION OF HYDROGEN ON A NICKEL (100) SURFACE

The adsorption of hydrogen on a clean Ni (100) single crystal surface was studied in UHV between 120 K and 500 K means of LEED, thermal desorption spectroscopy and work function measurements. H2 chemisorption occurs in two atomic states ßy and ßz with maximum desorption rates (at Saturation) at 318 K and 348 K. The activation energy for desorption was determined to be 20 (± 2) kcal/mole for the ß1-state and 23 (± 1) kcal/mole for the ß2-state, in excellent agreement with previous results. Saturation at 120 K corresponds to a coverage of 0.9 (± 20%). The initial sticking probability is 0.06 (± 0.01) and the rate of adsorption follows second order kinetics. No additional LEED spots arose from the H overlayer, indicating the absence of long-range order even at 120 K. The work function results clearly show the existence of two different binding states: Whereas the ß2-population gives rise to a linear increase by 100 meV (at 120 K) the ßi-state leads to a decrease of Acp to a final value of 30-40 meV. © 1979, Walter de Gruyter. All rights reserved.