Abstract
We study the gas-surface dynamics of O2 at Ag(111) with the particular objective to unravel whether electronic non-adiabatic effects are contributing to the experimentally established inertness of the surface with respect to oxygen uptake. We employ a first-principles divide and conquer approach based on an extensive density-functional theory mapping of the adiabatic potential energy surface (PES) along the six O2 molecular degrees of freedom. Neural networks are subsequently used to interpolate these grid data to a continuous representation. The low computational cost with which forces are available from this PES representation allows then for a sufficiently large number of molecular dynamics trajectories to quantitatively determine the very low initial dissociative sticking coefficient at this surface. Already these adiabatic calculations yield dissociation probabilities close to the scattered experimental data. Our analysis shows that this low reactivity is governed by large energy barriers in excess of 1.1 eV very close to the surface. Unfortunately, these adiabatic PES characteristics render the dissociative sticking a rather insensitive quantity with respect to a potential spin or charge non-adiabaticity in the O2–Ag(111) interaction. We correspondingly attribute the remaining deviations between the computed and measured dissociation probabilities primarily to unresolved experimental issues with respect to surface imperfections.
Highlights
Of the O2 gas-surface dynamics at the Ag(111) surface, for which low adsorption probabilities have been reported experimentally [10, 14]
Notwithstanding, on Ag(100) recent molecular dynamics (MD) calculations based on an ab initio six-dimensional (6D) potential energy surface (PES) showed that the comparable absence of O2 dissociation on that surface can be fully explained within an adiabatic picture [17]
Our simulations are based on a divide-and-conquer approach [21,22,23], in which first an accurate PES is constructed from first-principles and, classical MD calculations are performed using a continuous representation of this PES to describe the molecule–surface interaction
Summary
Of the O2 gas-surface dynamics at the Ag(111) surface, for which low adsorption probabilities have been reported experimentally [10, 14]. Notwithstanding, on Ag(100) recent molecular dynamics (MD) calculations based on an ab initio six-dimensional (6D) potential energy surface (PES) showed that the comparable absence of O2 dissociation on that surface can be fully explained within an adiabatic picture [17]. There, the system is characterized by large dissociation barriers of about 1.1 eV that appear when the molecule is close to the surface. This motivates a similar ab initio investigation for Ag(111), with the objective to settle the question of to what extent the low reactivity on the Ag(111) surface is really a signature of electronic non-adiabaticity or only the result of large dissociation barriers as found for Ag(100). The remaining deviations of the computed sticking curve to the measurements for Ei < 1 eV are more likely a result of unresolved issues in the experimental data, e.g. with respect to surface imperfections as discussed by the experimentalists [14, 19]
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