Inelastic electron tunneling spectroscopy and microscopy with a scanning tunneling microscope revealed two vibrational modes showing a decrease in conductance at $\pm 82.0$ and $\pm 38.3\mathrm{mV}$ sample bias for single oxygen molecules chemisorbed on the fourfold hollow sites of Ag(110) surface at 13 K. The spatial distribution of the vibrational intensities exhibited ${\pi }_g$-orbital (perpendicular to surface) symmetry of ${\mathrm{O}}_2$ with the molecular axis along the [001] direction. These results are attributed to resonant inelastic electron tunneling.

• Received 27 April 2000

DOI:https://doi.org/10.1103/PhysRevLett.85.1914