Gas phase H atoms abstract D adatoms on metal surfaces either along a direct abstraction path (d-ABS:H+Dad→HD), or along an indirect abstraction path (i-ABS:H+D→D2). Similar ABS reactions on Si surfaces are also known to take place. However, the dependence of d- and i-ABS on surface temperature Ts appears different among the metal and Si surfaces systems, suggesting their mechanisms are different. So far, the so-called hot atom (HA)—mediated ABS mechanism has been proposed on the metal surfaces [A], while the hot complex (HC)-mediated ABS mechanism has been proposed on the Si surfaces [B]. The HC mechanism is generally based on a first- or second-order kinetics for the d- or i-ABS pathways, respectively. In addition, the HA pathways are expected to be less affected by Ts [C]. It was reported that the i-ABS path on the metal surfaces shows up a feature that the reaction does not greatly depend on Ts. Yet, it is unclear whether it is not still affected seriously by Ts below 100 K. In this report we examine dependence of both the d- and i- ABS pathways on Ts on a Ru(0001) surface in the low temperature region where no recombination thermal desorption occurs. The experimental apparatus consists of a three stage differentially pumped atom beam line, an ultrahigh vacuum reaction chamber equipped with an AES, an Ar+ ion sputter gun, QMS, and a RAIRS. The Ru(0001) surface is cleaned by the repeated cycles of Ar+ ion sputtering and thermal annealing. Residual carbons are finally removed by annealing the crystal in an oxygen atmosphere. The sample is cooled down to 60 K by heat conduction from a liquid He cryostat. Upon admission of H atoms onto the D-terminated Ru(0001) surface we find D2 rate decreases at 60 K but we find considerable increase in the D2 rate above 100 K. This somewhat strong Ts dependence of the i-ABS pathway may suggest that the ABS reactions is not only the HA mechanism on the Ru(0001) surface.
[A]: S.Wehner and J.Kuppers: J.Chem.Phys., 108(1998), 3353.
[B]: F.Rahman, M.Kuroda, T.Kiyonaga, F.Khanom, H.Tsurumaki, S.Inanaga and A.Namiki: J.Chem.Phys., 121(2004), 3221.
[C]: J.Harris and B.Kasemo: Surf.Sci., 105(1981), L281. |