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 ßi 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 /?i-state and 23 ( ± 1) kcal/mole for the /^-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 longrange order even at 120 K. The work function results clearly show the existence of two different binding states: Whereas the /^-population gives rise to a linear increase by 100 meV (at 120 K) the ß\-state leads to a decrease of A <p to a final value of 30—40 meV.