Nils Bartels

The Dynamics of Molecular Interactions and Chemical Reactions at Metal Surfaces: Testing the Foundations of Theory

We review studies of molecular interactions and chemical reactions at metal surfaces, emphasizing progress toward a predictive theory of surface chemistry and catalysis. For chemistry at metal surfaces, a small number of central approximations are typically made: (a) the Born-Oppenheimer approximation of electronic adiabaticity, (b) the use of density functional theory at the generalized gradient approximation level, (c) the classical approximation for nuclear motion, and (d) various reduced-dimensionality approximations. Together, these approximations constitute a provisional model for surface chemical reactivity. We review work on some carefully studied examples of molecules interacting at metal surfaces that probe the validity of various aspects of the provisional model.

Observation of orientation-dependent electron transfer in molecule–surface collisions

Significance How molecules point in space—that is, their spatial orientation—determines how they interact with their environment. Exchange of energy, photons, and particles as well as chemical reactions are all elementary processes that depend on orientation. Electron transfer reactions are of particular interest because of their importance in a remarkably wide range of phenomena. In this work, we examine electron transfer reactions at surfaces, which control the change of oxidation state in surface chemistry, a critical factor explaining catalytic activity and selectivity. We report a strong orientation dependence for vibrational relaxation of a diatomic molecule colliding with a metal surface, an energy transfer process driven by electron transfer. These observations represent a challenge to modern theories of surface chemistry. Molecules typically must point in specific relative directions to participate efficiently in energy transfer and reactions. For example, Förster energy transfer favors specific relative directions of each molecule’s transition dipole [Förster T (1948) Ann Phys 2(1-2):55–75] and electron transfer between gas-phase molecules often depends on the relative orientation of orbitals [Brooks PR, et al. (2007) J Am Chem Soc 129(50):15572–15580]. Surface chemical reactions can be many orders of magnitude faster than their gas-phase analogs, a fact that underscores the importance of surfaces for catalysis. One reason surface reactions can be so fast is the labile change of oxidation state that commonly takes place upon adsorption, a process involving electron transfer between a solid metal and an approaching molecule. By transferring electrons to or from the adsorbate, the process of bond weakening and/or cleavage is initiated, chemically activating the reactant [Yoon B, et al. (2005) Science 307(5708):403–407]. Here, we show that the vibrational relaxation of NO—an example of electronically nonadiabatic energy transfer that is driven by an electron transfer event [Gadzuk JW (1983) J Chem Phys 79(12):6341–6348]—is dramatically enhanced when the molecule approaches an Au(111) surface with the N atom oriented toward the surface. This represents a rare opportunity to investigate the steric influences on an electron transfer reaction happening at a surface.

Dynamical steering in an electron transfer surface reaction: Oriented NO(v = 3, 0.08 < Ei < 0.89 eV) relaxation in collisions with a Au(111) surface

We report measurements of the incidence translational energy dependence of steric effects in collisions of NO(v = 3) molecules with a Au(111) surface using a recently developed technique to orient beams of vibrationally excited NO molecules at incidence energies of translation between 0.08 and 0.89 eV. Incidence orientation dependent vibrational state distributions of scattered molecules are detected by means of resonance enhanced multiphoton ionization spectroscopy. Molecules oriented with the N-end towards the surface exhibit a higher vibrational relaxation probability than those oriented with the O-end towards the surface. This strong orientation dependence arises from the orientation dependence of the underlying electron transfer reaction responsible for the vibrational relaxation. At reduced incidence translational energy, we observe a reduced steric effect. This reflects dynamical steering and re-orientation of the NO molecule upon its approach to the surface.

Observation of direct vibrational excitation in gas-surface collisions of CO with Au(111): a new model system for surface dynamics

We report vibrational excitation of CO from its ground (v = 0) to first excited (v = 1) vibrational state in collision with Au(111) at an incidence energy of translation of E(I) = 0.45 eV. Unlike past work, we can exclude an excitation mechanism involving temporary adsorption on the surface followed by thermalization and desorption. The angular distributions of the scattered CO molecules are narrow, consistent with direct scattering occurring on a sub-ps time scale. The absolute excitation probabilities are about 3% of those expected from thermal accommodation. The surface temperature dependence of excitation, which was measured between 373 and 973 K, is Arrhenius-like with an activation energy equal to the energy required for vibrational excitation. Our measurements are consistent with a vibrational excitation mechanism involving coupling of thermally excited electron-hole pairs of the solid to CO vibration.

Controlling an electron-transfer reaction at a metal surface by manipulating reactant motion and orientation.

The loss or gain of vibrational energy in collisions of an NO molecule with the surface of a gold single crystal proceeds by electron transfer. With the advent of new optical pumping and orientation methods, we can now control all molecular degrees of freedom important to this electron-transfer-mediated process, providing the most detailed look yet into the inner workings of an electron-transfer reaction and showing how to control its outcome. We find the probability of electron transfer increases with increasing translational and vibrational energy as well as with proper orientation of the reactant. However, as the vibrational energy increases, translational excitation becomes unimportant and proper orientation becomes less critical. One can understand the interplay of all three control parameters from simple model potentials.

Production of a beam of highly vibrationally excited CO using perturbations.

An intense molecular beam of CO (X(1)Σ(+)) in high vibrational states (v = 17, 18) was produced by a new approach that we call PUMP - PUMP - PERTURB and DUMP. The basic idea is to access high vibrational states of CO e(3)Σ(-) via a two-photon doubly resonant transition that is perturbed by the A(1)Π state. DUMP -ing from this mixed (predominantly triplet) state allows access to high vibrational levels of CO (X(1)Σ(+)). The success of the approach, which avoids the use of vacuum UV radiation in any of the excitation steps, is proven by laser induced fluorescence and resonance enhanced multi-photon ionization spectroscopy.