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Abstract
We present a combined experimental and theoretical study of NO(v = 3 → 3, 2, 1) scattering from a Au(111) surface at incidence translational energies ranging from 0.1 to 1.2 eV. Experimentally, molecular beam-surface scattering is combined with vibrational overtone pumping and quantum-state selective detection of the recoiling molecules. Theoretically, we employ a recently developed first-principles approach, which employs an Independent Electron Surface Hopping (IESH) algorithm to model the nonadiabatic dynamics on a Newns-Anderson Hamiltonian derived from density functional theory. This approach has been successful when compared to previously reported NO/Au scattering data. The experiments presented here show that vibrational relaxation probabilities increase with incidence energy of translation. The theoretical simulations incorrectly predict high relaxation probabilities at low incidence translational energy. We show that this behavior originates from trajectories exhibiting multiple bounces at the surface, associated with deeper penetration and favored (N-down) molecular orientation, resulting in a higher average number of electronic hops and thus stronger vibrational relaxation. The experimentally observed narrow angular distributions suggest that mainly single-bounce collisions are important. Restricting the simulations by selecting only single-bounce trajectories improves agreement with experiment. The multiple bounce artifacts discovered in this work are also present in simulations employing electronic friction and even for electronically adiabatic simulations, meaning they are not a direct result of the IESH algorithm. This work demonstrates how even subtle errors in the adiabatic interaction potential, especially those that influence the interaction time of the molecule with the surface, can lead to an incorrect description of electronically nonadiabatic vibrational energy transfer in molecule-surface collisions.
Highlights
A quantitative understanding of interactions between molecules and surfaces in microscopic detail is important for a variety of chemical processes at surfaces, many of which are central to heterogeneous catalysis
A cursory inspection of the resonance enhanced multiphoton ionization (REMPI) spectra shows that the branching ratio between vibrational relaxation into v = 2, 1 and survival in v = 3 changes with incidence energy
We have compared experimentally determined incidence-energy dependent vibrational relaxation branching ratios for NO(v = 3 → 3, 2, 1) scattering off a Au(111) surface to three different kinds of first-principles simulations: (1) adiabatic molecular dynamics, (2) molecular dynamics with electronic friction, and (3) molecular dynamics with independent electron surface hopping on a density functional theory (DFT) derived Newns-Anderson Hamiltonian
Summary
A quantitative understanding of interactions between molecules and surfaces in microscopic detail is important for a variety of chemical processes at surfaces, many of which are central to heterogeneous catalysis. The energy exchange between surface degrees of freedom and molecular vibration is of particular interest, as the vibrational motion is most closely related to molecular dissociation, that is, to chemical reaction. The Born-Oppenheimer approximation fails, and the molecular vibration can directly couple to electronic degrees of freedom. Such nonadiabatic coupling between molecular vibration and electron-hole pair excitation of the solid can have significant or even dominant influence on vibrational energy transfer.. The theoretical picture that was developed explains the strong vibrational damping by a transient population of the molecular affinity level, which is lowered in energy and broadened as the molecule comes close to the metal surface.. CO molecules adsorbed on metal surfaces such as Cu, Pt, or Ru have vibrational lifetimes on the order of picoseconds, compared to millisecond lifetimes observed for CO adsorbed on NaCl. The theoretical picture that was developed explains the strong vibrational damping by a transient population of the molecular affinity level, which is lowered in energy and broadened as the molecule comes close to the metal surface. The vibrational lifetimes could be reproduced by electronic friction (EF) theory, which describes the dissipation of vibrational energy by frictional forces that involve energy exchange with the electronic degrees of freedom of the metal.
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