C-O 1.15 (at), He II-ultraviolet photoelectron spectroscopy of CO on Cu (111) yields two peaks at EF -8.5 eV and EF -11.6 eV, at a coverage of θ = 1/3 (Figure ). According to these reports, CO primarily resides on the atomic site, with some bridging sites occupied. The experiment cannot readily resolve which peak corresponds to σ-and which to π-character, so the figure does not them. Nevertheless the similarity with Pt (111) in orbital decomposition of the DOS indicates the CO levels order in the same way. Indeed in most respects the Pt and Cu cases are very similar: the peaks at EF -8.5 and EF -11.6 eV most likely originate from a confluence of contributions of atomic and bridging/hollow sites (Figure ). This can be seen by the way the theoretical peaks straddle the PE data, depending on which site CO resides. Further, the position of unoccupied the 2π* peak depends on coverage (3.6 eV for θ = 1/3; 12 and 3.35 eV for θ = ½.) 11 For θ = ½, this has been attributed to an estimated occupation ratio of 13:12 atomic:bridging sites).