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The effect of CO chemisorption on the surface magnetism and unfilled electronic structure of Ni(llO) is investigated by spin-polarized inverse-photoemission spectroscopy. A saturation in the reduction of the unfilled minority-spin d density of states is observed near 0.5-monolayer CO coverage and attributed to a reduction in the Ni-atom magnetic moments. Transitions into the CO w* band are also observed with an intensity that increases nearly linearly with coverage. No transference of spin polarization from the Ni substrate to the CO IT* is found. PACS numbers: 79.60.Gs, 75.50.Cc Because there is strong coupling between the valence electrons in metal atoms of a solid, it follows that adsorption of a molecule on a metal surface should have an effect not only on the electronic structure of substrate atoms directly involved in the chemisorption bond, but on the subsurface and nearestneighbor atoms as well. Various theoretical studies of chemisorption support this view, 1 "" 3 and the wellknown sensitivity of surface states to small amounts of contamination provide experimental evidence for this "through-metal effect" on the surface electronic structure. In ferromagnetic substrates, surface magnetism also exhibits a high sensitivity to contamination. 4 " 8 Magnetism, when viewed as a macroscopic phenomenon, has its basis in the long-range ordering of the individual magnetic moments of the atoms, whereas the basis for the magnitude of the moments is derived from details of the ferromagnet's valence electronic structure. Early studies 4 ** 6 on the effect of chemisorption on magnetism were not able to distinguish whether the chemisorption-induced reduction in surface magnetism is due to a disruption of the longrange ordering of the magnetic moments or an actual reduction in the moments themselves. Only recently have there emerged experiments of sufficient detail to show the correlated effect that chemisorption has on the substrate magnetic and electronic structure. 7,8 In this Letter, we present the results of measurements on the effect of chemisorption of CO on the surface magnetic and electronic structure of Ni(110) using spin-polarized inverse-photoemission spectroscopy (SPIPES). These measurements indicate that the decrease in surface magnetism that is induced by CO chemisorption is a result of a reduction in the magnetic moments (or net spin) of the atoms rather than a reduction of the exchange coupling leading to a disordering of otherwise unchanged moments.