Abstract
We measured absolute probabilities for vibrational excitation of NO(v = 0) molecules in collisions with a Au(111) surface at an incidence energy of translation of 0.4 eV and surface temperatures between 300 and 1100 K. In addition to previously reported excitation to v = 1 and v = 2, we observed excitation to v = 3. The excitation probabilities exhibit an Arrhenius dependence on surface temperature, indicating that the dominant excitation mechanism is nonadiabatic coupling to electron-hole pairs. The experimental data are analyzed in terms of a recently introduced kinetic model, which was extended to include four vibrational states. We describe a subpopulation decomposition of the kinetic model, which allows us to examine vibrational population transfer pathways. The analysis indicates that sequential pathways (v = 0 → 1 → 2 and v = 0 → 1 → 2 → 3) alone cannot adequately describe production of v = 2 or 3. In addition, we performed first-principles molecular dynamics calculations that incorporate electronically nonadiabatic dynamics via an independent electron surface hopping (IESH) algorithm, which requires as input an ab initio potential energy hypersurface (PES) and nonadiabatic coupling matrix elements, both obtained from density functional theory (DFT). While the IESH-based simulations reproduce the v = 1 data well, they slightly underestimate the excitation probabilities for v = 2, and they significantly underestimate those for v = 3. Furthermore, this implementation of IESH appears to overestimate the importance of sequential energy transfer pathways. We make several suggestions concerning ways to improve this IESH-based model.
Highlights
Molecular-level studies of the fundamental energy transfer processes at gas-solid interfaces are crucial for a predictive understanding of such important processes as heterogeneous catalysis, etching and corrosion
Semi-quantitative agreement was obtained between experimental data and an independent-electron surface hopping (IESH) based model, over a wide range of surface temperatures, ܶௌ, and incidence energies for vibrational excitation observed in the nitric oxide (NO)(=ݒ0→1,2)/Au(111) system.[6]
They all peak at the specular angle (3.3o from the surface normal) and are well described by a narrow cosଵ.ଵ(ߠ + 3.3°) distribution
Summary
Molecular-level studies of the fundamental energy transfer processes at gas-solid interfaces are crucial for a predictive understanding of such important processes as heterogeneous catalysis, etching and corrosion. There is a growing body of experimental and theoretical evidence that when molecules interact with metallic surfaces, the adiabatic (Born-Oppenheimer) approximation breaks down and molecular vibration can couple to electron-hole pair (EHP) excitations of the metal 1. Extensive data is available for electronically nonadiabatic coupling of nitric oxide (NO) with various metal surfaces, including work on both vibrational relaxation 2 and excitation 3. Several theoretical approaches were successful in at least qualitatively describing electronically nonadiabatic vibrational relaxation 4. Obtaining quantitative agreement is more challenging using a first-principles approach 5. Semi-quantitative agreement was obtained between experimental data and an independent-electron surface hopping (IESH) based model, over a wide range of surface temperatures, ܶௌ, and incidence energies for vibrational excitation observed in the NO(=ݒ0→1,2)/Au(111) system.[6]
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