Laterally varying surface concentrations associated with the oscillatory oxidation of carbon monoxide on a Pt(l 10) surface were imaged by photoemission electron microscopy. Depending on the applied conditions, a large variety of spatiotemporal patterns were observed that are characteristic for the nonlinear dynamics of reaction-diffusion systems.PACS numbers: 68.35.-p, 82.65.-i Chemical reactions far from equilibrium may exhibit, even if operated under stationary continuous-flow conditions, various phenomena of temporal and spatial selforganization. Effects belonging in these categories have already been widely investigated, both experimentally and theoretically, with homogeneous reactions in solution, in particular, with the famous Belousov-Zhabotinskii (BZ) reaction. 1 Corresponding studies with heterogeneous reactions occurring at well-defined single-crystal surfaces were initiated only more recently. 2 Among the latter, the catalytic oxidation of CO on a Pt(llO) surface was found to exhibit a very rich variety of oscillatory kinetics. The underlying microscopic mechanism has been explored in great detail, enabling satisfactory theoretical modeling of the temporal behavior. 2,3 Generally, the occurrence of temporal variations of the concentrations of the species involved in the reaction implies that these also vary spatially, unless local differences are eliminated by "stirring," i.e., rapid convection. With the BZ reaction (and other homogeneous reactions in solution), the resulting spatiotemporal concentration patterns can easily be made visible by the associated color differences. More refined techniques are, however, required to image local variations of surface properties. For example, recent attempts to image patterns associated with the CO oxidation reaction include the application of scanning low-energy electron diffraction, 4 as well as scanning photoemission microscopy 5 (SPM) and photoemission electron microscopy (PEEM). 6 The latter technique is particularly versatile due to its high temporal (-10 ms) and lateral (-0.1 j/m) resolution, and a newly designed instrument of this type 7 was applied in the present work to image an unprecedented rich variety of spatiotemporal patterns associated with the oscillatory reaction of catalytic CO oxidation on Pt( 110).The principle of the PEEM technique consists in illuminating the sample surface with UV light from a deuterium lamp (with its cutoff near 6.9-eV photon energy) and to image the lateral intensity distribution of the photoemitted electrons through a system of electrostatic lenses after amplification by a channel plate onto a "back-view" fluorescence screen. The intensity of photoemitted electrons depends on the (local) work function O which in turn is affected by the dipole moments of the adsorbate complexes.In the current investigation, Oof a clean Pt(l 10) surface increases 0.3 eV when saturated with CO, and 0.5 eV through oxygen chemisorption. Therefore, areas covered by O a d appear dark in the images, while those covered by CO a d are brigh...

The electron energy loss and x-ray absorption spectra of two phases of titanium dioxide-rutile and anatase-are reported. Although the nearest-neighbour environments of titanium and oxygen in both these structures are very similar, noticeable differences are observed between the electron energy loss and the x-ray absorption spectra. Attempts to model these differences using real-space multiple-scattering calculations are reasonably successful at the Ti K and 0 K edges. We propose an interpretation of the Ti L2,3 edge, notably for the presence of a previously unobserved splitting on the L, edge.

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