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 ${\mathrm{C}}_2{\mathrm{H}}_2$ easily occupies both configurations because the difference between their reaction barriers is small, whereas ${\mathrm{C}}_2{\mathrm{H}}_4$ 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 ${\mathrm{C}}_2{\mathrm{H}}_2$ and ${\mathrm{C}}_2{\mathrm{H}}_4.$

• Received 6 January 2003

DOI:https://doi.org/10.1103/PhysRevB.69.075303