Vibrational Energy Dissipation in Noncontact Single-Molecule Junctions Governed by Local Geometry and Electronic Structure.

The vibrational dynamics of adsorbate molecules in single-molecule junctions depend critically on the geometric structure and electronic interactions between the molecule and the substrate. Vibrations, excited mechanochemically or by external stimuli, dissipate energy into substrate electrons and phonons. Energy dissipation leads to the broadening of spectral lines, vibrational lifetimes, and the coupling between molecular and substrate phonons. It affects molecular manipulation, giving rise to nanoscale friction, and contributes to scanning probe and surface spectroscopy signals. We present an approach to disentangle adsorbate vibrational dynamics in noncontact junctions by employing Density Functional Theory, machine learning, and nonadiabatic molecular dynamics. Focusing on the CO-functionalized Cu surfaces representing a single-molecule junction, a widely studied system in scanning probe and energy dissipation experiments, we reveal strong vibrational mode specificity governed by the interplay of electron-phonon and phonon-phonon coupling. Electron-phonon relaxation rates vary by 2 orders of magnitude between modes and sensitively depend on the tip-substrate geometry. We find evidence of a weak nonadditive effect between both energy dissipation channels, where electron-phonon coupling enhances phonon-phonon coupling. Our predicted vibrational lifetimes agree with infrared spectroscopy and helium scattering experiments. Finally, we outline how our findings can inform and enhance spectroscopy and scanning probe experiments.