TOTAL-ENERGY CALCULATIONS FOR ACETYLENE ADSORPTION AND DECOMPOSITION ON SI(100)-2X1

The adsorbed states and vibrational properties of acetylene on Si(100) -2 X 1 have been studied using a molecular cluster model and the atom- superposition and electron-delocalization molecular-orbital theory. Ac etylene is determined to di-sigma bond to the surface dimer sites. Its C-C bond order decreases to between single and double, while its C-H bonds are only slightly weakened. The bonding angles HCC, HCSi, and Si CC of adsorbed C2H2(a) are close to ethylenic values. Both the acetyle ne pi donation to the surface and substrate backdonation to the acetyl ene pi orbital result from the equilibrium di-sigma bonding geometry. The decomposition paths and activation barriers of C2H2(a) on Si(100) have been analyzed in detail. The calculated vibrational properties o f C2H2(a) and C2H2-induced species confirm the results of a recent hig h-resolution electron-energy-loss study.