Electron-mediated phonon-phonon coupling drives the vibrational relaxation of molecules at metal surfaces

There is no doubt that the efficiency in creating low-energy electronic excitations must be responsible for decreasing the lifetime of vibrationally excited adsorbates from the milisecond scale in semiconductors to the picosecond scale in metals. The absence of an energy threshold for creating electronic excitations in the latter facilitates the vibrating molecule to transfer part of its energy to the substrate by creating electron-hole pairs excitations, irrespective of the value of the energy with which the molecule vibrates. Even if this physical picture is clear and well established, present first principles theories that accounts for this electron-vibration coupling are still unable to explain the fast vibrational relaxation that is reported experimentally.

Schematic figure of the electron-mediated phonon-phonon coupling process of the internal stretch mode of CO on Cu(100). The internal stretch mode (red) couples to low energy frustrated rotation and frustrated translation modes, as well as to the modes consisting of the joint motion of CO and Cu surface atoms (blue)

In this letter, Novko et al. demonstrate that the electron-mediated intermode vibrational coupling is the missing piece in our understanding of the controversial nonadiabatic vibrational relaxation. Electrons, besides coupling directly to the vibrating molecules, drive the otherwise forbidden coupling between modes with very different energies. It is the combination of these two effective relaxation mechanisms that finally reconcile the theoretical predictions with experiments. Additionally, this study elucidates the underlying cause of the elusive temperature dependence measured in the high-energy mode relaxation rates. Considering that the relaxation rates are fundamental building blocks for the theoretical description of dynamical process at metal surfaces, this new theory will enrich our understanding of nonadiabaticy not only in vibrating adsorbates, but even more generally in many other surface reactions.