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

## TL;DR

This paper studies how vibrations in single-molecule junctions dissipate energy through interactions with the substrate, using theory and simulations to reveal how geometry and electronic structure influence this process.

## Contribution

A novel approach combining DFT, machine learning, and nonadiabatic molecular dynamics to analyze vibrational energy dissipation in noncontact single-molecule junctions.

## Key findings

- Vibrational mode specificity is governed by electron–phonon and phonon–phonon coupling.
- Electron–phonon relaxation rates vary by two orders of magnitude depending on tip–substrate geometry.
- Electron–phonon coupling enhances phonon–phonon coupling in a weak nonadditive effect.

## Abstract

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.