Coupling of Molecular Vibrational Modes across a Vacuum Barrier

While the flexibility of a CO-functionalized tip in scanning probe methods is at the origin of extraordinarily high resolution in imaging, its impact on inelastic electron tunneling spectroscopy has remained unexplored to date. Here, coupled vibrational modes of a CO molecule bonded to the tip of a scanning tunneling microscope and a single melamine isomer on Cu(001) are studied in a combined experimental and theoretical work. Decreasing the distance between CO and the adsorbed isomer causes a substantial tilt of the CO molecule, which reduces the symmetry of the junction geometry and thereby lifts the degeneracy of the CO frustrated translation modes. The entailed blueshift of the CO vibrational mode depends on the isomer and reflects the different local interactions between the molecules. The findings of this work are applicable to a general class of molecular adsorbates and pave the way for establishing inelastic electron tunneling spectroscopy as a useful characterization tool in surface physics and chemistry.