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Abstract

Photoemission electron microscopy (PEEM) is a well known technique which was developed along with early electron microscopes. A new version of this microscope has recently been developed which is compatible with ultrahigh vacuum (UHV) conditions and allows real-time observation of the surface work function with a working resolution of 1 μm. The contrast mechanism of this microscope is based on local differences in electronic work function. The adsorption of reactants in many cases leads to a work function change in the surface. During the catalytic oxidation of CO on platinum-group metals at oxygen pressures above 10^-5 Torr, the formation of CO, oxygen and mixed reactive islands can be observed. In this study, the reaction kinetics were followed along with the formation of different reactive structures. The transition from one reactive phase to another occurred via island formation and reactive fronts. The nucleation occurred at a countable number of extended defects (several μm in diameter) which had their own characteristic dynamics. The growth process of islands on Pt(111) were followed by PEEM and were related to the macroscopic reaction rate measurements of bistability. The PEEM measurements showed clearly that there was a range of parameters where the reaction exhibits bistability and this corresponded to the pressure region determined from macroscopic experiments. For oscillating conditions the extended defects acted as pacemakers for the formation of target patterns and spirals.