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) � 2 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.
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