Adsorption kinetics of acetylene and ethylene on Si(001)

The reaction of acetylene and ethylene on the Si(001) surface is investigated by first-principles density-functional calculations within the generalized-gradient approximation. We have identified the two different reaction pathways that result in adsorptions on top of a single dimer and across the ends of two adjacent dimers in the same dimer row. Our calculated energy profile of the reaction path shows that C2H2 easily occupies both configurations because the difference between their reaction barriers is small, whereas C2H4 occupies mainly the former configuration because of a relatively larger barrier for formation of the latter configuration. This result provides an explanation for a subtle difference in the adsorption structures of C2H2 and C2H4.