Unified description of H-atom–induced chemicurrents and inelastic scattering

Abstract

The Born-Oppenheimer approximation (BOA) provides the foundation for virtually all computational studies of chemical binding and reactivity, and it is the justification for the widely used "balls and springs" picture of molecules. The BOA assumes that nuclei effectively stand still on the timescale of electronic motion, due to their large masses relative to electrons. This implies electrons never change their energy quantum state. When molecules react, atoms must move, meaning that electrons may become excited in violation of the BOA. Such electronic excitation is clearly seen for: (i) Schottky diodes where H adsorption at Ag surfaces produces electrical "chemicurrent;" (ii) Au-based metal-insulator-metal (MIM) devices, where chemicurrents arise from H-H surface recombination; and (iii) Inelastic energy transfer, where H collisions with Au surfaces show H-atom translation excites the metal's electrons. As part of this work, we report isotopically selective hydrogen/deuterium (H/D) translational inelasticity measurements in collisions with Ag and Au. Together, these experiments provide an opportunity to test new theories that simultaneously describe both nuclear and electronic motion, a standing challenge to the field. Here, we show results of a recently developed first-principles theory that quantitatively explains both inelastic scattering experiments that probe nuclear motion and chemicurrent experiments that probe electronic excitation. The theory explains the magnitude of chemicurrents on Ag Schottky diodes and resolves an apparent paradox--chemicurrents exhibit a much larger isotope effect than does H/D inelastic scattering. It also explains why, unlike Ag-based Schottky diodes, Au-based MIM devices are insensitive to H adsorption.M ost theoretical studies of atoms and molecules interacting with metal surfaces are based on the Born-Oppenheimer approximation (BOA) (1). However, a growing number of examples have been found where electronic and nuclear degrees of freedom are strongly coupled in violation of the BOA (2-6). H-atom interactions at metals offer a remarkable opportunity to test non-BOA theories against experiment, since H-adsorptioninduced chemicurrent experiments (7-14) offer a direct measure of electronic excitation and H-atom inelastic scattering experiments (15) directly probe nuclear motion. In chemicurrent experiments, exothermic H interactions like adsorption and recombination produce hot electrons that pass over a potential barrier to be collected. Hence, the magnitude of the chemicurrent is dependent on both the reaction-induced production of hot electrons and the likelihood of transmission over the barrier. To reduce uncertainties associated with barrier transmission, the ratio of hydrogen-and deuterium-induced chemicurrent is often measured--H-induced chemicurrents are typically two to five times larger than those from D atoms (7,(11)(12)(13). H-atom surface scattering experiments yield H-atom translational energy loss distributions. The importance of H-atom translation to electronic exc...

Inelastic H and D atom scattering from Au(111) as benchmark for theory

“…This explains the observed scattering angle dependency of the energy loss: At specular angle, single bounce events are observed with higher probability, while at surface normal, multibounce events dominate. As the energy loss increases with the number of collisions, 18 the average energy loss increases with decreasing scattering angle.…”

“…The large energy loss is due to an efficient energy transfer to electron-hole-pair excitations. [17][18][19] In the following, three aspects of the scattering experiments are discussed in detail: First, the influence of the incidence parameters of the H atom beam, second the influence of the azimuthal orientation of the crystal, and third the isotope effect.…”

“…According to the theoretical model, the most probable energy loss approximately represents the average energy loss for single bounce events. 18 In the above example, the most probable energy losses are 380 meV for H and 500 meV for D, which results in ∆ED,e = ∆EH,e = 340 meV and ∆EH,p = 1/4 ⋅ ∆ED,p = 40 meV. Therefore, this simple analysis yields that 89% of the transferred energy goes to electronic excitations in the case of H and 68% in the case of D, close to the values predicted by the theoretical simulation.…”

“…16 Recently, we performed inelastic scattering experiments of H and D atoms from metal surfaces. [17][18][19] A comparison between an insulator and a metal surface showed that a large amount of translational energy is transferred to electronic excitations of the metal surface in the scattering event, explaining the high adsorption probabilities. Kandratsenka et al developed a theoretical model based on molecular dynamics (MD) simulations on a global full-dimensional potential energy surface (PES) to simulate the inelastic scattering experiments.…”

Symmetry-Adapted High Dimensional Neural Network Representation of Electronic Friction Tensor of Adsorbates on Metals

“…30 Given its simplicity and efficiency, combined with high-dimensional DFT-based PESs or on-the-fly electronic structure calculations, the LDFA-based MDEF model has enabled a firstprinciples description of the non-adiabatic energy loss in various processes of atoms/molecules interacting with metal surfaces, 9,[32][33][34][35][36][37][38][39][40][41] sometimes yielding close to quantitative agreement with experiments. 18,[42][43] Despite these successes, LDFA has been continuously questioned in describing non-adiabatic interactions between molecules and metal surfaces, because the atomic friction coefficients intrinsically lack the directional dependence and molecular anisotropy that arises from the electron-nuclear coupling of a many-electron system described in a realistic potential. 35,[44][45][46][47][48] In principle, the full-rank EFT can be obtained by the first-order time-dependent perturbation theory (TDPT) based on KS orbitals of DFT that fully accounts for the electronic structure of molecule-surface system.…”