Covalency in Oxygen Chemisorption as probed by X-ray Absorption

X-ray absorption spectra at the O 1s edge are presented for O(2×2)/Ni(111), O(2×1)/Cu(110), and O/Si(111)-(7×7). The spectra yield information on the distribution of O 2p and np state character in the unoccupied states. The data are compared with that for oxides. We conclude that there is considerable covalency in the chemisorptive bond and that, even for transition metals, this arises mainly from interaction between the O 2p levels and the substrate sp bands.