CO Adsorption on Thin MgO Films and Single Au Adatoms: A Scanning Tunneling Microscopy Study

Abstract

The adsorption behavior of CO on a 2 ML thin MgO/Ag(001) film is investigated with scanning tunneling microscopy and spectroscopy at 5 K. Depending on the tip state, single CO molecules are imaged as Gaussian depressions or ring-like features on the oxide surface. The preferred CO adsorption sites are identified as the Mg2+ positions next to an oxide step edge. The deposition of single Au atoms followed by CO exposure gives rise to the formation of monocarbonyl species on the MgO surface. Their vibrational properties are explored by inelastic electron-tunneling spectroscopy. The acquired second-derivative spectra are dominated by a symmetric peak/dip structure at ±50 mV, which is assigned to the frustrated rotation of Au-bound CO molecules.