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Abstract

Two new optical imaging methods with submonolayer surface sensitivity are applied together to investigate pattern formation of
adsorbate concentrations on surfaces during heterogeneously catalyzed reactions. Ellipso-microscopy for surface imaging (EMSI) is
based on an ellipsometric effect, reflection anisotropy microscopy (RAM) on the different reflectivity properties of nonisotropic surfaces. During the CO oxidation reaction on a Pt(110) surface, features such as front propagation, target patterns and spiral waves are
observed with EMSI and RAM, similar as in prior investigations with the photo emission electron microscope. The new methods
are promising steps towards bridging the pressure gap between the "pure" surface science approach with its ultra-high vacuum
(UHV) conditions and the "ordinary" atmospheric environment of chemical reactions, for instance in an automobile's catalytic converter. Of course, when increasing the pressure, one additional parameter has to be considered: the reaction is no longer isothermal. Being a highly exothermic reaction, already at pressures of 10^-2 mbar 0₂ and 490 K, an increase of the sample temperature of several Kelvin could be observed. This increase is generated only on the reactive areas or oxygen covered areas, and
gives rise, at least for very thin probes, to temperature patterns, observable with a sensitive infrared camera. By utilizing three
different optical methods, the pattern formation can now be studied from regions clearly governed by diffusion and/or gas phase coupling mechanism to regions where the generated temperature is the dominating factor.