Identification of electronic transitions localized at a single oxygen atom adsorbed on Cu(110)-(2×1) surface

We have identified electronic transitions tightly localized at a single oxygen atom adsorbed on the Cu(110)-(2×1) surface, by means of scanning tunneling microscope (STM) light emission spectroscopy. The emission spectrum measured with the STM tip located directly over the oxygen atom shows two peaks corresponding to states located ∼200 and ∼300 meV above the Fermi level of Cu. These peaks are identified with unoccupied states localized at a single oxygen atom.