UC Irvine

Inelastic Electron Tunneling Spectroscopy with the Scanning Tunneling Microscope (STM-IETS) is a powerful technique used to characterize vibration and spin at the single molecule level. While IETS lacks hard selection rules, historically it has been assumed that vibrational overtones are rarely seen or even absent. Here we provide direct experimental evidence that the hindered rotation overtone excitation of CO molecules on Ag(110) can be detected with STM-IETS by isotope substitution. We also show that the anharmonicity of the overtone excitation can be characterized and compared between unique adsorption sites, and find evidence of anisotropy in the vibrational anharmonicity for CO adsorbed on the [1 1 0] step edge.