APPLICATION OF PHOTOEMISSION ELECTRON-MICROSCOPY TO THE STUDY OF SURFACE-REACTIONS

Photoemission electron microscopy (PEEM) is a well known technique whi ch was developed along with early electron microscopes. A new version of this microscope has recently been developed which is compatible wit h ultrahigh vacuum (UHV) conditions and allows real-time observation o f the surface work function with a working resolution of 1 mum. The co ntrast mechanism of this microscope is based on local differences in e lectronic work function. The adsorption of reactants in many cases lea ds to a work function change in the surface. During the catalytic oxid ation of CO on platinum-group metals at oxygen pressures above 10(-5) Torr, the formation of CO, oxygen and mixed reactive islands can be ob served. In this study, the reaction kinetics were followed along with the formation of different reactive structures. The transition from on e reactive phase to another occurred via island formation and reactive fronts. The nucleation occurred at a countable number of extended def ects (several mum in diameter) which had their own characteristic dyna mics. The growth process of islands on Pt(111) were followed by PEEM a nd were related to the macroscopic reaction rate measurements of bista bility. The PEEM measurements showed clearly that there was a range of parameters where the reaction exhibits bistability and this correspon ded to the pressure region determined from macroscopic experiments. Fo r oscillating conditions the extended defects acted as pacemakers for the formation of target patterns and spirals.