Acetylene adsorption on Si(111): Molecular orbital theory

Abstract

Using an idealized unreconstructed structure model for the Si(111) surface and the ASED-MO theory, vibrational properties are calculated which corroborate the HREELS vibrational study of Yoshinobu et al. of acetylene adsorbed on the (7 × 7) reconstructed surface. Acetylene is predicted to bind with an energy of 2 eV to a di-δ site. The CC bond stretches 0.17 Å to a value between double and single bond lengths and the HCC, HCSi and SiCC angles are close to ethylenic values at 117°, 119°, and 124°, respectively. Both acetylene π donation to the surface and surface radical state back-donation to the acetylene π z ∗ orbital occur. A parallel study of acetylene adsorption on the hexagonal form of Si shows a 0.04 eV increase in the acetylene adsorption energy but no other changes.