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IBM Journal of Research and Development  
Volume 22, Number 3, Page 260 (1978)
Surface Science
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Chemisorption of Ethane on W(111)

by H. F. Winters
Ethane is chemisorbed on W(111) with a sticking probability of ≈0.003. The carbon Auger spectrum at saturation co verage exhibits a two-peak structure similar to that for graphite, while the LEED (low energy electron diffraction) pattern is almost identical to that obtained for an atomically clean surface. Heating the surface to ≈773 K causes desorption of hydrogen and changes th e carbon Auger spectrum to a three-peak structure similar to that for tungsten carbide. After annealing, the LEED pattern is affected in different ways depending on the precise conditions, but it may in certain circumstances almost disappear. Exposure to ethylene produces a similar sequence of events. A large kinetic isotope effect is observed with the ratio of the sticking probabilities [S(C2H6)/S(C2D6), W(111), T = 300 K] being ≈3. A similar ratio is measured for tungsten at T = 2500 K. These data suggest that chemisorption is dissociative in nature, probably involving the reaction C2H6 → C2H5* + H → subsequent steps. Heating of the surface completely dissociates the adsorbed gas, leaving adsorbed carbon and gas phase hydrogen. In analogy with previous work on methane, we believe the large isotope effect suggests that the initial dissociation reaction is dominated by the tunneling of a hydrogen atom through a potential barrier. The implications of this conclusion for other saturated molecules will be discussed.
Related Subjects: Chemistry and chemical engineering; Electron trap characterization; Films; Physical chemistry