Multimode vibrational activation and energy transfer in single-molecule CO hopping on Pd(111)

We report a vibrationally induced single-molecule hopping of carbon monoxide (CO) on Pd(111) by action spectroscopy with a scanning tunneling microscope (STM-AS). The observed hopping yields reveal vibrational thresholds at 96, 124, 142, and 230 meV, which correspond to high-order overtones of the metal-carbon (M-C) stretch mode and the fundamental C-O stretch mode. Morse potential fitting enables quantitative estimation of anharmonicity and supports overtone-driven activation. Comparison with previous studies on Pd(110) shows that the significantly higher reaction yield on Pd(111) arises from enhanced anharmonic coupling between high-frequency modes and the frustrated translational mode, the reaction coordinate for lateral hopping. This work emphasizes the role of site-dependent anharmonic interactions in energy transfer, with overtone excitations as an available pathway. Our findings offer new insights into multimode vibrational activation mechanisms in surface reactions and highlight a means of manipulating molecular motion at the atomic scale.