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  • 1
    Digitale Medien
    Digitale Medien
    Influence of extended interactions on the surface dynamics in the catalytic CO oxidation on Pt single crystal substrate (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 8672-8679 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The simulation of kinetic oscillations and dynamics of CO chemical fronts propagating through a Pt(100) substrate by the use of cellular automaton techniques, is reexamined with an extended interacting neighborhood model, that allows for a temperature dependence of the reaction constants. These new cellular automaton rules account for the structural phase transformations of the Pt substrate, the reaction kinetics of the adsorbed phase, and diffusion of adsorbed species. In addition it provides a richer variety of surface patterns formation. Specifically, we consider the effect of lateral interactions between adsorbed particles, this is taken into account our simulations through the definition of a site-dependent reaction constant, that is dependent of the state of the metal. This is achieved by considering an extended neighborhood of next-nearest-neighbors and beyond. For CO–CO and O–O attractive interactions, the oscillation amplitude increases as the interaction energy increases. It is shown herein that the presence of strong attractive lateral interactions in reacting adsorbate can lead to the spontaneous formation of traveling waves. The diffracting properties of the waves moving in the media with different composition (one part clean and other containing inert sites) are also investigated. It is shown that beyond a critical coverage, waves traveling from a clean part of the crystal to a partially covered surface are adsorbed by the media, destroying their coherence. Finally, it is shown that appropriate initial conditions can also lead to the formation of double stable spirals.© 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.481469
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  • 2
    Digitale Medien
    Digitale Medien
    Effect of inert sites on the kinetic oscillations in the catalytic CO oxidation on Pt(100) (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 8617-8625 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The effect of inert sites in the global oscillations in the oxidation of CO on Pt(100) for both random and clustered inert sites is simulated by use of the cellular automaton technique. The cellular automaton rules account for the structural phase transformations of the Pt substrate, the reaction kinetics of the adsorbed phase and diffusion of adsorbed species. The introduction of a fraction θd of inert sites reduces the extent of the oscillatory region on the bifurcation diagram. The effect of added impurities on the transition from oscillatory to CO poisoned state is found to be quite different depending on the initial position on the bifurcation diagram. Points located near the rightmost branch on the bifurcation diagram will reach inert state through a very abrupt transition, whereas points located far from it undergo a much softer transition. Clustered impurities tend to soften the transition, and higher values of θd are needed to trigger the transition to the poisoned state. The study of a mean-field model without the oscillatory kinetics explains some of the basic features of the effect of increasing densities of impurities on the catalytic surface, for instance the dependence of the location of the transition point θd on the adsorption rate of CO. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.477528
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  • 3
    Digitale Medien
    Digitale Medien
    Dynamics of front propagation in the catalytic CO oxidation on Pt(100) (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 8119-8128 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The dynamics of CO and O chemical fronts propagating through a Pt(100) substrate are simulated using cellular automaton techniques. The cellular automaton accounts for the adsorption of both molecular species CO and O2 on a platinum substrate, as well as the reaction kinetics between these species and the diffusion of CO on the substrate. The influence of inert sites (defects) is also considered. The chemical front is generated in two different situations: In the first one, the sample is initially covered by CO and the front is generated by depositing a layer of O atoms. The system is then subject to more O2 molecules which can adsorb and react with the other CO molecules and the interface moves as the reaction proceeds. It is found that the fluctuation of the interface in a clean sample follows the Kardar–Parisi–Zhang equation and, in this case, a microscopic description in terms of a stochastic process is also proposed. In a second situation, the adsorption and diffusion of CO are allowed in addition to the previous reaction mechanisms. In this case, it is found that the growth obeys the scaling relation, w(L,t)∼Lαf(t/Lz), and our numerical results tend to indicate a continuous dependence of the exponents β and α on the diffusion of CO. The reflecting and diffracting properties of the waves moving in the media with different composition (one part clean and the other containing inert sites) are also investigated herein. The results agree well with the experimental work of Asakura et al. [Surf. Sci. 374, 125 (1997)] and in particular it is found that the chemical concentration waves during heterogeneously catalyzed reactions obey Snell's law. © 1999 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.478715
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  • 4
    Digitale Medien
    Digitale Medien
    Kinetic oscillations in the catalytic CO oxidation on Pt(100) with adsorbed impurities (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 10353-10360 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The effect of inert sites (random impurities) in the global oscillations in the oxidation of CO on Pt(100) is studied analytically via a stability analysis of the underlying mean field model. The analysis shows that the introduction of a fraction θd of inert sites can reduce considerably the extent of the oscillatory region on the bifurcation diagram. The study of a mean field model explains some of the basic features that arise upon increasing the density of impurities on the catalytic surface, and it allows the analytical calculation of the bifurcation diagram. The analytical solution of the model reproduces well cellular automata simulations. In particular, we show the existence of a critical concentration of inert sites at which the oscillatory region vanishes and oscillations are no longer possible, due to the inability of the lattice to reach the minimum local concentration of CO required to trigger the surface reconstruction mechanism. We have evaluated this maximum defect concentration in the mean field approximation to be exactly θd*=1−θCO+, where θCO+ is the minimum CO concentration required to trigger the (hex) to (1×1) Pt substrate transition. For the set of parameters chosen, we find here θd*=0.6, which is in perfect agreement with our earlier simulation findings. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1322656
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  • 5
    Digitale Medien
    Digitale Medien
    Clonal analysis in hybrids between Drosophila melanogaster and Drosophila simulans (1994)
    Springer
    Development genes and evolution 204 (1994), S. 112-117 
    Details
    ISSN: 1432-041X
    Schlagwort(e): Drosophila melanogaster ; Drosophila simulans ; Hybrids ; Clonal analysis ; Lhr
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Biologie
    Notizen: Abstract We have analysed the viability of cellular clones induced by mitotic recombination in Drosophila melanogaster/D. simulans hybrid females during larval growth. These clones contain a portion of either melanogaster or simulans genomes in homozygosity. Analysis has been carried out for the X and the second chromosomes, as well as for the 3L chromosome arm. Clones were not found in certain structures, and in others they appeared in a very low frequency. Only in abdominal tergites was a significant number of clones observed, although their frequency was lower than in melanogaster abdomens. The bigger the portion of the genome that is homozygous, the less viable is the recombinant melano-gaster/simulans hybrid clone. The few clones that appeared may represent cases in which mitotic recombination took place in distal chromosome intervals, so that the clones contained a small portion of either melanogaster or simulans chromosomes in homozygosity. Moreover, Lhr, a gene of D. simulans that suppresses the lethality of male and female melanogaster/simulans hybrids, does not suppress the lethality of the recombinant melanogaster/simulans clones. Thus, it appears that there is not just a single gene, but at least one per tested chromosome arm (and maybe more) that cause hybrid lethality. Therefore, the two species, D. melanogaster and D. simulans, have diverged to such a degree that the absence of part of the genome of one species cannot be substituted by the corresponding part of the genome of the other, probably due to the formation of co-adapted gene complexes in both species following their divergent evolution after speciation. The disruption of those coadapted gene complexes would cause the lethality of the recombinant hybrid clones.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00361105
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  • 6
    Digitale Medien
    Digitale Medien
    Characterization of the smoothness and curvature of a marginal function for a trust-region problem (1999)
    Springer
    Mathematical programming 84 (1999), S. 123-135 
    Details
    ISSN: 1436-4646
    Schlagwort(e): Key words: marginal or value function – perturbation or sensitivity analysis – trust regions Mathematics Subject Classification (1991): 65U05, 90C20, 90C30, 90C31
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik , Mathematik
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s10107980027a
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  • 7
    Digitale Medien
    Digitale Medien
    Bilevel and multilevel programming: A bibliography review (1994)
    Springer
    Journal of global optimization 5 (1994), S. 291-306 
    Details
    ISSN: 1573-2916
    Schlagwort(e): Bilevel (two level) ; three level and multilevel programming ; static Stackelberg problems ; hierarchical optimization ; minimax problems
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Mathematik
    Notizen: Abstract This paper contains a bibliography of all references central to bilevel and multilevel programming that the authors know of. It should be regarded as a dynamic and permanent contribution since all the new and appropriate references that are brought to our attention will be periodically added to this bibliography. Readers are invited to suggest such additions, as well as corrections or modifications, and to obtain a copy of the LaTeX and BibTeX files that constitute this manuscript, using the guidelines contained in this paper. To classify some of the references in this bibliography a short overview of past and current research in bilevel and multilevel programming is included. For those who are interested in but unfamiliar with the references in this area, we hope that this bibliography facilitates and encourages their research.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01096458
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  • 8
    Digitale Medien
    Digitale Medien
    A new technique for generating quadratic programming test problems (1993)
    Springer
    Mathematical programming 61 (1993), S. 215-231 
    Details
    ISSN: 1436-4646
    Schlagwort(e): Test problem generation ; quadratic programming ; global optimization ; large-scale optimization
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Informatik , Mathematik
    Notizen: Abstract This paper describes a new technique for generating convex, strictly concave and indefinite (bilinear or not) quadratic programming problems. These problems have a number of properties that make them useful for test purposes. For example, strictly concave quadratic problems with their global maximum in the interior of the feasible domain and with an exponential number of local minima with distinct function values and indefinite and jointly constrained bilinear problems with nonextreme global minima, can be generated. Unlike most existing methods our construction technique does not require the solution of any subproblems or systems of equations. In addition, the authors know of no other technique for generating jointly constrained bilinear programming problems.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01582148
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