Vibrations of O on stepped Pt(111)

First-principles calculations identify edge-bridging O adatoms as the source of the $560{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ vibrational loss observed for step-adsorbed O on Pt(533), and by implication, of the scalloped appearance, in scanning tunneling microscopy of O-saturated $A$-type island edges on Pt(111). Further calculations for O/Pt(221) provide an opportunity to confirm that O adatoms prefer upper-terrace fcc hollows at a $B$-type step.