Ab initio description of non-adiabatic effects in dissociative adsorption at surfaces

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A predictive materials science modeling based on microscopic understanding requires a thorough knowledge of all underlying elementary processes at the atomic scale. The (dissociative) adsorption of individual gas phase molecules at metal surfaces is such an elementary process that is of crucial relevance for any application involving surfaces exposed to realistic gas environments, with heterogeneous catalysis forming just one prominent example. Unfortunately, our present understanding of this fundamental process is even for a most simple, but ubiquitous diatomic molecule like 02 very shallow. Lacking reliable and resolved information about the interaction of an impinging molecule with the surface electronic structure (summarized in form of a socalled potential-energy surface (PES)), empirical theories have invoked controversial scenarios to account for experimentally accessible, integral kinetic quantities like the sticking probability at the surface. Among these scenarios is the frequently discussed role of non-adiabatic effects in the dissociation, i.e. a possible violation of the Born-Oppenheimer approximation and electron distributions that are not able to follow the nuclear motion instantaneously. Clarifying insight into this matter can only come from firstprinciples theories like density-functional theory (DFT), which involve a reliable, though approximate description of the electronic structure of the system. Based exclusively on the adiabatic DFT-PES, such theories have in the past been predominantly developed for H2 dissociation at metal surfaces, where non-adiabatic effects apparently play no role. Recent methodological developments have now also enabled the calculation of diabatic PESs within constrained DFT approaches. Correspondingly, we propose a first systematic investigation addressing the real importance of non-adiabatic effects in 02 dissociation. To unambiguously pin down the role played by these effects, this requires to concomitantly tackle a number of other, not yet fully understood issues like the required accuracy in the description of electron correlation or the influence of surface mobility. Focusing at the dynamics of 02 dissociation at a representative set of surfaces, the emerging trend picture over the periodic table is expected to contribute to a first comprehensive understanding of the (dissociative) adsorption of this most important molecule, that may also give some guidance on the dissociation of other widespread diatomics like N2, NO or CO.