Using surface X-ray scattering, a (3 x 1) Si(111) reconstruction induced by chemisorption of CaF2 was studied. This reconstruction consisted of a parallel array of zig-zag Si adatom chains oriented along <1¯10>. The orbitals on the Si chain atoms re-hybridized into px and py along the surface to form σ bonds, and parallel spz orbitals perpendicular to the surface to give additional π bonding. This basic structure, with no dangling bonds at only ⅓ monolayer adsorbate coverage, could explain the occurrence of (3 x 1) reconstructions. Parallel Adatom Chains on Si(111) - a Chemisorption-Induced Surface Reconstruction. G.C.L.Wong, C.A.Lucas, D.Loretto, A.P.Payne, P.H.Fuoss: Physical Review Letters, 1994, 73, 991-4