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The goal of the Surface Chemical Dynamics Program is to elucidate the underlying physical processes that determine the products (selectivity) and yield (efficiency) of chemical transformations relevant to energy-related chemistry on catalytic and nanostructured surfaces. Achieving this end requires understanding the evolution of the reactant-molecule/surface complex as molecules adsorb, bonds dissociate, surface species diffuse, new bonds form and products desorb. The pathways and time scales of these processes are ultimately determined by a multidimensional potential energy surface that is a function of the geometric and electronic structures of the surface and the reactant, product, intermediate and transition-state molecular and atomic species.

To understand these processes, our group employs a battery of methods to measure the energetics and timescales of energy and charge transfer in the molecule/surface complex. Excitation of the complex is brought about either by preparing the reactants in a molecular beam or exciting adsorbates with photons. The response of the system is probed by measuring sticking coefficients, rovibrational-state-, angle-, and time-resolved molecular desorption distributions, electronic energy distribution dynamics, photoinduced desorption dynamics and adsorbate vibrational energy relaxation dynamics. In addition we are developing surface nanostructuring methods, including physical vapor and size-selected cluster deposition that can be coupled to these powerful surface chemical dynamics probes. By exercising temporal control over the excitation and spatial control over the surface we aim to gain detailed insight into the dynamical links between the structure and function of reactive surfaces.

surface chemical dynamics illustration

A molecule’s perspective on surface chemistry. This cartoon schematically depicts, at the molecular level, the richness of the phenomena involved in the transformation of reactants to products at the surface of a material. A molecule may scatter off the surface, experiencing no or some finite degree of energy exchange with the surface. Alternatively, molecule-surface energy transfer can lead to accommodation and physical adsorption or chemical adsorption. In some cases, physisorption is a precursor to chemisorption, and in some cases, bond dissociation is required for chemisorption. Charge transfer plays a critical role in some adsorption processes. Once on the surface, the adsorbed intermediate may diffuse laterally with a temperature dependent rate, sampling surface features including adatoms, vacancies and steps. They may become tightly bound to a defect site. Various adsorbed intermediates may meet, either at defect sites or at regular lattice sites, and form short-lived transition state structures and ultimately product molecules. Finally, products desorb from the surface with a temperature dependent rate, imparting some fraction of the energy of the association reaction to the surface. The goal of surface chemical dynamics is to identify the roles, quantify the rates and understand the physics of these various mechanistic steps that comprise the surface chemical transformation.

The Surface Chemical Dynamics Program, part of the BNL Chemical Physics Program which is supported by the Chemical Physics Research Program of the Division of Chemical Sciences, Biosciences, and Geosciences of the Office of Basic Energy Sciences of the Office of Science under contract No. DE-AC02-98CH10886 with the U.S. Department of Energy.

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