Parallel Adatom Chains on Si(111): A Chemisorption-Induced Surface Reconstruction

Using surface x-ray scattering, we have studied a (3 × 1) Si(111) reconstruction induced by chemisorption of Ca${\mathrm{F}}_2$. This reconstruction consists of a parallel array of zigzag Si adatom chains oriented along . The orbitals on the Si chain atoms rehybridize into $p_x$ and $p_y$ along the surface to form $\sigma$ bonds, and parallel ${\mathrm{sp}}_z$ orbitals perpendicular to the surface to give additional $\pi$ bonding. This basic structure, with no dangling bonds at only ⅓ monolayer adsorbate coverage, may explain the occurrence of (3 × 1) reconstructions among a diverse range of Si-adsorbate systems.