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Abstract
The knowledge of elementary rates is necessary in modeling reactions of technological and environmental importance. In light of this, a novel visible optical probe has been developed to monitor the rates of elementary processes on metal surfaces at submonolayer coverages. The technique relies on the reflectivity changes that are induced by adsorption of molecules. The small changes to the reflectivity caused by physisorption are due to the presence of a polarizable overlayer on an unperturbed substrate. The two order of magnitude larger reflectivity changes observed for chemisorption arise from perturbations to the electronic structure of the substrate.
The adsorbate induced reflectivity changes were correlated with the coverage and overlayer ordering for carbon monoxide, acetylene, and oxygen on Cu(100). The observed frequency dependence and the linear correlation of the reflectivity changes with coverage for carbon monoxide and acetylene are consistent with a surface electron scattering model. The lack of dependence of the reflectivity changes on adsorbate ordering suggest that the electron scattering is primarily inelastic involving vibrational excitation of the overlayer. For oxygen, a nonlinear correlation with the coverage is observed. This can be justified within a statistical adsorption model that shows that a single oxygen atom perturbs the reflectivity over a 46(6) Å2 region; this implies that the electronic structure of the surface is altered over a region larger than the first coordination shell of the oxygen atom.
The technique is used to study several elementary processes. The adsorption kinetics of carbon monoxide and acetylene have been measured and follow a precursor mediated adsorption mechanism. The desorption kinetics of carbon monoxide have also been measured. The reactions of acetylene on Cu(100) have been studied in detail. Three reaction channels have been identified: desorption, trimerization to benzene, and isomerization. As an extension of the optical technique to more complex situations, we have used it to measure the surface isomerization reaction rate. This work demonstrates that it is possible to quantitatively measure the rates of elementary surface processes, even in the presence of more than one surface species with a simple visible optical technique.
