Ordered oxygen overlayers on Cr(100) observed by leed and photoemission

Chemisorbed oxygen atoms on Cr(100) induce strong O(2p) derived surface resonances which are studied by angle resolved photoemission. Well ordered structures are observed after annealing (300°C). In the submonolayer range (θ_O < 1) a study of the symmetry and dispersion of the O(2p) derived features shows the two-dimensional Bloch character associated with either a c(2 × 2)-O surface at low coverages (θ_O?0.25) or a (1 × 1)-O structure at high coverages (θ_O?0.9). When combined with LEED observations and work function data this study indicates that both structures coexist around θ_O = 0.5 and chemisorbed oxygen is probably incorporated into the fourfold hollow sites. At θ_O > 1, the onset of oxidation is clearly shown in the valence band and core level spectra and the data support the existence of a thin spinel-like oxide layer.