Spin-resolved inverse-photoemission study of Ni(001) and its chemisorption

Abstract

Results from an angle-resolved spin-polarized inverse photoelectron spectroscopy (ARSPIPES) study of Ni(001), c(2×2)O/Ni(001), and c(2×2)S/Ni(001) are presented. Inverse-photoemission spectra for the clean Ni(001) surface are in good agreement with previous work. A minority-spin character is found for a direct radiative transition into an unoccupied 3d band. No definitive spin dependence is found for radiative transitions into the unoccupied 4sp band, a potentially unresolved crystal-induced surface state, or an image-potential surface state. A chemisorbed c(2×2) oxygen overlayer does not appreciably affect the measured spin dependence or spectral intensity of the Ni 3d radiative transition. Little evidence is found for an oxygen-induced unoccupied state above the Fermi level at the Γ¯’ point of the surface Brillouin zone. The persistent spin asymmetry for the Ni 3d transition suggests that any magnetization decrease induced by oxygen is probably confined to the Ni surface layer. In contrast to c(2×2)O/Ni(001), a c(2×2) sulfur overlayer increases the spectral intensity just above the Fermi level, and drastically reduces the measured spin dependence of the ARSPIPES spectra. These observations are tentatively assigned to a sulfur-induced majority-spin state above the Fermi level at Γ¯’. The image-potential state remains fixed in energy upon c(2×2) sulfur chemisorption. This behavior is in contrast to that observed for c(2×2)O/Ni(001), and indicates that the Ni(001) image-potential state energy need not shift with adsorbate-induced variations in the work function.