Chemical Bonding of Unique CO on Fe(100)

At low coverage, CO molecules are known to preferentially occupy the hollow sites of Fe(100) with considerably inclined molecular orientations. This CO configuration serves as the precursor state of CO dissociation, which is particularly important in several important catalytic reactions. In this study, we present a unique bonding picture of the precursor state from the spin, charge, and orbital perspectives. From the spin and orbital views, we show the antiferromagenetic nature of the adsorbate-metal coupling, where 2 pi magnetism prevails with a dominant spin-down channel. However, contrasting tendencies are found for the two 1 pi orbitals in two orthogonal directions: the 1 pi orbital in the vertical plane loses its symmetry, whereas the other 1 pi orbital remains intact with a preserved symmetry. The 1 pi symmetry in the vertical plane favors the 1 pi -> 2 pi* excitation mechanism owing to the partial opening up of the 1 pi symmetry. In the charge perspective, we have identified a charge transfer mechanism involving the local structural Fe-I(C)-C-O motif, in which the surface slightly charges the adsorbate with additional partial electrons located at the surface Fe atoms bonded to the carbon end, whereas the charges of the metallic atoms beneath the Fe-I(C)-C-O motif are found to be depleted. In both the adsorbate and metal sides, the depletion of s electrons serves as a good measure of orbital repulsion and delocalization. Interestingly, the carbon and oxygen ends exhibit contrasting electron affinity with the metal surface: the carbon end is attractive, whereas the oxygen end is repulsive in terms of the contrasting charge rearrangement in the bonded metallic atoms.