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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5327-5336 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ultraviolet-photochemistry of molecularly adsorbed oxygen on Pd(111) has been studied using pulsed laser light with 6.4 eV photon energy. Three processes occur upon irradiation: desorption of molecular oxygen, conversion between adsorption states, and dissociation to form adsorbed atomic oxygen. By using time-of-flight spectroscopy to detect the desorbing molecular oxygen and post-irradiation thermal desorption spectroscopy (TDS) to characterize the adsorbate state, a detailed picture of the photochemical processes is obtained. The data indicate that the O2 molecules desorbing with low translational energies from the saturated surface as well as the conversion of adsorbed molecules between binding states are induced by the photoinduced build-up of atomic oxygen on the surface. Analysis of a proposed reaction model reproduces the observed data and yields detailed rates. Polarization analysis indicates that the photochemical processes are initiated by electronic excitations of the substrate.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 3154-3169 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 4609-4619 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: UV-laser irradiation (hν=6.4 eV and 5.0 eV) of the water bilayer adsorbed on a Pd(111) surface leads to molecular desorption and to conversion of the adsorbed state as manifested in thermal desorption spectra. The latter effect is attributed to photodissociation of water on the surface. Time-of-flight measurements show that water molecules desorb with a translational energy of about 600 K for both photon energies indicating a nonthermal process. While desorption is largely suppressed with adsorbed multilayers, conversion within the first layer still proceeds. The dependence of the desorption yield on angle of incidence and polarization of the light reveals substrate excitations as the dominant primary step. A strong variation of cross sections with isotopic substitution is observed. This is interpreted as evidence for the operation of a mechanism involving excitation onto an isotope-independent excited potential energy surface followed by rapid deexcitation to the ground state so that, of the total number of species excited, only a small mass dependent fraction actually fragments or desorbs.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 1509-1510 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ultraviolet-laser irradiation (6.4 eV and 5.0 eV) of the first layer of water adsorbed on a Pd(111) surface at 90 K leads to desorption of H2O and to conversion of the adsorbed state as manifested in the thermal desorption spectra. The latter effect is attributed to photodissociation of water on the surface. Time-of-flight measurements show that water molecules desorb with the same translational energy of about 600 K for both photon energies. While desorption is suppressed with adsorbed multilayers, conversion within the first layer still proceeds.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 1202-1214 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A systematic study of spiral waves in a realistic reaction-diffusion model describing the isothermal CO oxidation on Pt(110) is carried out. Spirals exist under oscillatory, excitable, and bistable (doubly metastable) conditions. In the excitable region, two separate meandering transitions occur, both when the time scales become strongly different and when they become comparable. By the assumption of surface defects of the order of 10 μm, to which the spirals can be pinned, the continuous distribution of wavelengths observed experimentally can be explained. An external periodic perturbation generally causes a meandering motion of a free spiral, while a straight drift results, if the period of the perturbation divided by the rotation period is a natural number.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 2109-2126 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reaction of NO and CO on Pt(100) exhibits two branches of steady state production of N2 and CO2 and the occurrence of kinetic oscillations. This system was studied under steady flow conditions in the 10−6 mbar total pressure range using low-energy electron diffraction-(LEED), work function measurement, and mass spectrometry for determination of the reaction rate. These studies revealed that kinetic oscillations can only be initiated from one of the two stable reaction branches. Two separate existence regions were detected in which the oscillations are always damped. Oscillations can be very reproducibly excited by slight decreases in temperature. The 1×1(large-closed-square)hex phase transition of the surface structure was observed to take place only in one of the two regions of reaction rate oscillations. Its influence seems to be of minor relevance to the mechanism of oscillations as oscillations in one region occur on the surface that maintains a 1×1 structure. The experiments were modeled by a set of coupled differential equations based on knowledge about the elementary reaction steps. The model calculations reproduced the steady states of the reaction as well as the occurrence of kinetic oscillations in different ranges in excellent agreement with experimental observation. In the model, the phase transition also has no relevance for the oscillation mechanism. The occurrence of oscillations can be rationalized in terms of a periodic sequence of autocatalytic "surface explosions'' and the restoration of an adsorbate-covered surface. The damping, experimentally observed, is attributed to insufficient spatial coupling between different regions of the surface.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 8595-8604 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A two-variable Langmuir–Hinshelwood mechanism for isothermal CO oxidation on a catalytically active surface is presented. It shows bistability stemming from 2 cusp bifurcations, which can be obtained analytically for low pressure. Inclusion of CO diffusion on the surface leads to a system of partial differential equations, which exhibits nucleation and front propagation phenomena in the bistable region. While the line of equistability could with good accuracy be solved for analytically, the front velocities and critical radii for nucleation had to be determined numerically (using the method of heteroclinic orbits). Throughout the calculations the kinetics and rate constants for the CO oxidation on Pt(111) are used. Here the model can be reduced by adiabatic elimination of one variable (namely oxygen coverage) allowing a comparison to the exactly solved one-variable Schlögl model. Possible implications for future experimental work are briefly discussed.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 95 (1991), S. 8393-8402 
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 9248-9254 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dissociative chemisorption of nitrogen on clean and cesiated Ru(0001) surfaces has been studied using high-resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). N2 (at 300 K) chemisorbs dissociatively with a sticking coefficient of 2×10−6, independent of substrate temperature which was varied between 420 and 700 K. The saturation coverage is found at 0.5 monolayer. The energy of the N–Ru stretching vibration is 71 meV at the bare surface and 69 meV at the cesiated Ru(0001) surface. The activation energy for desorption is about 190 kJ/mol for small coverages. The kinetic data suggest the existence of an activation barrier in the entrance channel of adsorption. Preadsorption of 0.08 monolayer of Cs increases the sticking coefficient only by a factor of 1.3, and the maximum amount of adsorbed N is reduced due to blocking of adsorption sites through Cs.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 9977-9985 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamic behavior of elliptical front propagation and spiral-shaped excitation concentration waves associated with the catalytic oxidation of CO on a Pt(110)-surface was investigated by means of photoemission electron microscopy (PEEM). The properties of these patterns can be tuned through the control parameters, viz., the partial pressures of CO and O2 and the sample temperature. Over a wide range of control parameters the transition between two metastable states (COad and Oad covered surface) proceeds via nucleation and growth of elliptical reaction-diffusion (RD)-fronts. Front velocities and critical radii for nucleation are determined by the diffusion of adsorbed CO under reaction conditions. If at constant pO2, T the CO partial pressure is increased beyond a critical value a transition to qualitatively different dynamic behavior takes place. The elliptical fronts give way to oxygen spiral waves of excitation spreading across the CO-covered areas. For fixed experimental conditions a broad distribution of spatial wavelengths and temporal rotation periods was found. This effect has to be attributed to the existence of surface defects of μm-size to which the spiral tip is pinned. These data lead to a dispersion relation between the front propagation velocity and the wavelength, respectively, period. In addition, the dynamics of free spiral-shaped excitation waves was investigated under the influence of externally modulated temperature. Now the spiral starts to drift, resulting in distortion of the Archimedian shape and a pronounced Doppler effect.
    Type of Medium: Electronic Resource
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