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...

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Ultraviolet irradiation of NO2 adsorbed on top of a NO saturated Pd(111) surface causes the photodissociation of NO2/N2O4 and results in the desorption of NO molecules. This process has been studied using excitation energies between 3.5 and 6.4 eV. At a photon energy of 6.4 eV, a cross section of 3×10−18 cm2 is found. Using laser-induced fluorescence to detect the desorbed NO molecules, fully state-resolved data detailing the energy channeling into different degrees of freedom has been obtained. Two desorption channels are found, one characterized by nonthermal state populations, and one showing accommodation to the surface. The yield of the fast channel shows a marked increase above 4 eV photon energy. The slow channel is interpreted as being due to NO molecules which, after formation, undergo a trapping–desorption process. A polarization experiment indicates that the photodissociation is initiated by excitation of metal electrons rather than direct absorption by the adsorbate.

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The rate of catalytic carbon monoxide oxidation on a Pt(100) single crystal surface under isothermal, low-pressure conditions exhibits for certain ranges of parameters (O2 and CO partial pressures, temperature) sustained temporal oscillations whose mechanism had been explored in previous work. Coupling between reaction and diffusion leads to spatial pattern formation as manifested by patches with different work function on the intrinsically homogeneous surface. Imaging is performed by means of the novel technique of scanning photoemission microscopy. Typically, nuclei with dimensions of a few microns, as determined by the instrumental resolution, are formed spontaneously and expand with sharp fronts and velocities of about 0.5 mm/min (at 480 K) up to sizes ≥1 mm. Waves with even more extended fronts propagating with somewhat higher velocities across the sample surface are responsible for the occurrence of large amplitude temporal oscillations of the integral reaction rate.