Electron–hole pairs during the adsorption dynamics of O₂ on Pd(100): exciting or not?

GENERAL SCIENTIFIC SUMMARY Introduction and background. 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 non-adiabatic electronic excitation channel has been controversially discussed, as the absence of a band gap could favour an easy coupling to a manifold of electron–hole pairs of arbitrarily low energies.

Main results. 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. Despite the high Pd density of states at the Fermi level, the concomitant non-adiabatic energy losses 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.

Wider implications. Our showcase system underlines the necessity to also consider the high dimensionality of the surface dynamical process when assessing the excitation of electron–hole pairs and concomitant energy dissipation. On aluminium, i.e. a simple free electron metal, a highly non-adiabatic spin transition has been found to dominate the adsorption dynamics. Therefore, our suggested consequences for the O₂ spin dynamics contribute to elucidating the latter for a transition metal surface. These are of much greater interest in heterogeneous catalysis, and the adsorption of oxygen is a crucial elementary reaction step for oxidation reactions in many catalytic cycles.

Figure. Electron–hole pair excitations created by an O₂ molecule impinging side-on above a hollow site (h-para) as shown in the inset at the bottom. (a) Potential energy surface V_6D along the trajectory given by the reaction coordinate Q (neural network interpolation = black solid line, DFT input data = black circles), as well as projections of the spin density onto the two constituting oxygen atoms (O^A, O^B = dotted lines in shades of dark red, sum of O^A and O^B = light red solid line). (b) Evolution of reaction coordinate Q(t) and corresponding velocity along the trajectory. (c) Separate electron (at positive excitation energies) and hole (at negative excitation energies ω spectra P_ex,el ^σ(ω) and P_ex,ho ^σ(ω). (d) Total e–h pair spectrum P_ex ^σ(ω) together with resulting dissipated energies. All spectra are for a half round trip with energies ω relative to the Fermi energy. Both majority (↑, violet) and minority (↓, blue) spin channels are shown.