A KINETIC-STUDY OF THE INTERACTION OF GASEOUS H(D) ATOMS WITH D(H) ADSORBED ON NI(100) SURFACES

The kinetics of reactions which occur upon subjecting D(H) covered Ni( 100) surfaces with H(D) atom fluxes were investigated. At 120 K surfac e temperature in the H-->D-ad reaction HD and D-2 were observed as rea ction products, in the D-->H-ad reaction HD and H-2 were reaction prod ucts. As the reaction temperature was well below the hydrogen desorpti on temperature, a direct reaction step, like in the Eley-Rideal (ER) m echanism, is suggested to operate for HD production. However, the char acteristics of the HD formation kinetics observed in the present study contradict an essential element of the ER: mechanism the rate: of HD formation is not proportional to the surface coverage of the adsorbed reaction species D or H under impact of a flux of H or D atoms. Theref ore, a modification of the mechanistic description of atom/surface rea ctions seems necessary. This modification should allow for reaction pr oducts which are completely unaccounted for in the ER picture: D-2 fro m H-->D-ad and H-2 from D-->H-ad reactions. The observed strong isotop e effects in the reaction efficiencies support mechanisms in which the impinging gas phase atoms get trapped in the surface potential well p rior to reaction. (C) 1997 American Institute of Physics.