Electron-hole pairs during the adsorption dynamics of O2 on Pd(100) - Exciting or not?

TL;DR

This study investigates electronic excitations during O2 dissociation on Pd(100), finding that non-adiabatic electron-hole pair creation contributes minimally to energy loss, supporting a phonon-dominated heat dissipation model.

Contribution

It provides a detailed first-principles analysis of electron-hole pair excitations during O2 adsorption on Pd(100), highlighting the limited role of non-adiabatic electronic energy loss.

Findings

Electron-hole pair spectra vary significantly with molecular trajectories.

Non-adiabatic energy losses are less than 5% of chemisorption energy.

Electronic excitations support an adiabatic approximation for heat dissipation.

Abstract

During the exothermic adsorption of molecules at solid surfaces dissipation of the released energy occurs via the excitation of electronic and phononic degrees of freedom. For metallic substrates the role of the nonadiabatic electronic excitation channel has been controversially discussed, as the absence of a band gap could favour an easy coupling to a manifold of electronhole pairs of arbitrarily low energies. We analyse this situation for the highly exothermic showcase system of molecular oxygen dissociating at Pd(100), using time-dependent perturbation theory applied to first-principles electronic-structure calculations. For a range of different trajectories of impinging O2 molecules we compute largely varying electron-hole pair spectra, which underlines the necessity to consider the high-dimensionality of the surface dynamical process when assessing the total energy loss into this dissipation channel. Despite the high Pd density of states at the Fermi level, the concomitant non-adiabatic energy losses nevertheless never exceed about 5% of the available chemisorption energy. While this supports an electronically adiabatic description of the predominant heat dissipation into the phononic system, we critically discuss the non-adiabatic excitations in the context of the O2 spin transition during the dissociation process.