Surface Phonon-Mediated Molecular Diffusion via Vibration-Phonon Coupling at the Single-Molecule Level

Understanding the underlying mechanisms that induce molecular motion on solid surfaces is essential for elucidating elementary surface processes such as molecular diffusion and reactions. To date, several excitation pathways using electrons, heat, and light have been shown to induce the vibrational excitation of molecular adsorbates on surfaces. However, the contribution of substrate phonons to the behaviors of adsorbates has been underexplored despite its ubiquitous importance in various chemical processes. Here, we present evidence that surface phonons can actively induce molecular diffusion, as revealed by investigating the diffusion mechanism of CO on an ultrathin MgO film epitaxially grown on Ag(100). Using action spectroscopy with a scanning tunneling microscope and density functional theory calculations, we reveal a novel diffusion pathway mediated by surface phonons, along with the conventional vibrational excitation of the CO stretching mode. This newly observed diffusion pathway is identified as the Fuchs-Kliewer phonon mode of MgO, coupled with the CO-substrate stretching motion. These findings provide direct mechanistic insight into surface phonon-mediated molecular motion at the single-molecule level, offering a well-defined framework for understanding the vibration-phonon coupling in surface molecular dynamics.