Adsorbate-substrate vibrational modes of benzene on Ag(110) resolved with scanning tunneling spectroscopy

Using a low temperature scanning tunneling microscope, we have detected low energy adsorbate-substrate (external or frustrated) vibrational modes of b enzene molecules adsorbed on a Ag(110) surface. We demonstrate that such vi brations represent a fingerprint of the molecules' chemical state and envir onment; two different vibrational spectra are measured on molecules populat ing two different adsorption states. We also find that the distortion of th e adsorption geometry of the molecules may give rise to the excitation of a dditional (initially hidden) modes. Important differences in the spatial di stribution of the inelastic signal are also observed for these external mod es.