On the Nature of the Triphase Ordering.

It is shown that the model for oxygen chemisorption improves our understanding of the surface reaction. The disordered (2×1)-O precursor is identified as molecule adsorption in which the paired ^-1 rests on top of the surface along the close-packed direction. Meanwhile, ^-1 polarizes its rest neighbors, leading to the protruding domain boundaries. At higher coverage, (2×4)-4O and then (2×1)-2O develop due to ^-2 hybridization. ^-2 is located at a threefold hollow site and forms a tetrahedron with two Co atoms at the surface and one atom in the second layer. The lack of one atom of bonding is compensated for by an electron cloud of dipoles; namely, a virtual bond forms. The virtual bonding lowers the STM protrusions and narrows the antibonding band. [ABSTRACT FROM AUTHOR]