The structure of oxygen adsorption phases on Cu(100)

A study of the structure of the Cu(100) surface in the presence of adsorbed oxygen is presented based on new scanned energy mode O 1s photoelectron diffraction measurements in conjunction with qualitative LEED observations and O K-edge NEXAFS characterisation. The interpretation of these data is discussed in the context of the considerable body of (frequently conflicting) results from other techniques. The best characterised structural phase is the ( 2 × 2 )R45° −O structure and this is found to be best described by a missing-row model. Evidence for this model is derived from a variety of techniques, and some complementary quantitative details of the structure are derived from the present measurements and published quantitative LEED analyses, with excellent agreement between the techniques on the parameter to which both are sensitive, the O overlayer to metal substrate spacing. In this structure the oxygen atoms occupy sites quite close to coplanar with the top Cu atom layer. Both photoelectron diffraction and X-ray absorption spectra show that the local adsorption structure is different at lower coverages characterised by a “four-spot” LEED pattern; in this case there is evidence for chemisorbed oxygen atoms occupying more than one site or having substantial local disorder on the Cu(100) substrate. Some specific local site models of this phase are considered.