Reactive removal of unstable mixed NO+CO adlayers: Chemical diffusion and reaction front propagation

A lattice-gas model is developed to describe the reactive removal of a preadsorbed, mixed NO+COadlayer covering a Pt(100) surface, via reduction of NO with CO, and behavior of the model is analyzed. Since NO dissociation requires an adjacent empty site, the NO+CO covered surface constitutes an unstable steady state. The creation of vacancies leads NO dissociation, the reaction of CO with the O formed by dissociation, the subsequent creation of more vacancies, and thus the autocatalytic removal of the adlayer. The high mobility of most adspecies leads to an initial “disperse stage” of adlayer removal, characterized by an exponential increase in the number of highly dispersed vacancies. Thereafter follows a transition to a “reaction front propagation” stage of adlayer removal, where a chemical wave develops that propagates into the NO+CO covered region of the surface with roughly constant velocity, and leaves in its wake a surface populated only by excess reactant. We provide a suitable rate equation formulation for the initial disperse stage, but focus on a reaction-diffusion equation analysis of reaction front propagation, examining, in detail, behavior for long times where the front is nearly planar. We emphasize that it is necessary to incorporate the coverage-dependent and tensorial nature of chemical diffusion in the mixed adlayer. Both these features reflect the interference on the surfacediffusion of each adspecies by coadsorbed species. Thus, a key component of this work is the development of an appropriate treatment of chemical diffusion in mixed layers of several adspecies

Percolative diffusion of CO during CO oxidation on Pt(100)

During CO-oxidation on Pt(100), CO diffuses in a disordered environment produced by a complex pattern of reconstructed and unreconstructed regions of the substrate. Macroscopic diffusion of CO is effectively only possible on percolating 1X1-regions of the substrate. We treat the spatio-temporal behavior observed in this reaction system accounting in the simple way for the percolative nature of CO-diffusion. This is done via incorporation into the reaction-diffusion equations of a suitable chemical diffusion coefficient, exploiting ideas from the theory of transport in disordered media. We use these equations to analyze the propagation of reactive, O-rich pulses into a CO-covered 1X1-background

Monomer-dimer surface-reaction model: Infiuence of the dimer adsorption mechanism

We consider the monomer-dimer surface reaction without surface diffusion for various dimer adsorption mechanisms, described below. After a dimer impinges \u27\u27end on\u27\u27 at an empty site, its bottom atom remains there while its top atom searches N1 sites, either in a local neighborhood (N-local models), or randomly located on the surface (N-random models), to find a second empty site. If one is found, the dimer can then adsorb dissociatively. The N-local models have a reactive window of finite width in the relative impingement rates, bordered by poisoning transitions, whereas the N-random models exhibit true bistability. As N increases, the reactivity is either strictly or effectively confined to relative impingement rates close to the stoichiometric ratio. We precisely analyze the limiting behavior as N-\u3eoo

Hazards to golden-mantled ground squirrels and associated secondary hazard potential from strychnine baiting for forest pocket gophers

Radio telemetry and capture-recapture techniques were used to evaluate the hazards to golden-mantled ground squirrels (Spermophilus lateralis) from hand baiting with 0.5% strychnine-treated oats for western pocket gophers (Thomomys mazama) on conifer plantations in eastern Oregon. Toxicology data were collected on field-killed and caged ground squirrels and on caged mink (Mustela vison), great horned owls (Bubo virginianus), and red-tailed hawks (Buteo jamaicensis). Ground squirrel populations were reduced 50 to 75% following underground baiting for pocket gophers. Maximum amount of strychnine alkaloid found in cheek pouches and carcass of a field-killed golden-mantled ground squirrel was 2.88 mg. Mean amount of strychnine in carcasses was 0.35 mg; almost all occurred in the gut. The estimated LD50 for mink was 0.6 mg/kg. The lowest lethal dose for great horned owls and red-tailed hawks was 7.7 mg/kg and 10.2 mg/kg, respectively. The LD50 for owls and hawks was not determined. Long-term effects on golden-mantled ground squirrel populations and secondary hazard potential to owls and hawks were judged to be minimal. Wild mustelids as large as mink could be adversely affected by consuming the gut content of strychnine-killed golden-mantled ground squirrels

Hybrid treatment of spatio‐temporal behavior in surface reactions with coexisting immobile and highly mobile reactants

For surface reactions on single-crystal substrates which involve highly mobile adspecies, there is a vast separation in natural time and length scales. Adspecies hop rates can be many orders of magnitude larger than rates for other processes. Strong spatial correlations or ordering can exist on the atomic scale, while spatial pattern formation occurs on a macroscopic scale due to high diffusivity. An efficient analysis of such systems is provided by a hybrid treatment which we apply here to the monomer-dimer surface reaction model in the case of coexisting immobile dimer adspecies and highly mobile monomer adspecies. Specifically, we combine a mean-field treatment of the randomized mobile adspecies, and a lattice-gas description of the immobile adspecies. Monte Carlo simulations then reveal bistability and critical bifurcation phenomena, while precisely accounting for the influence of correlations in the immobile adspecies distribution. A corresponding analysis of the evolution of macroscopic spatial inhomogeneities is achieved through parallel simulation of the distributed macroscopic points with distinct correlated states and adspecies coverages. These simulations are appropriately coupled to describe diffusive mass transport of the mobile adspecies. In this way, we examine for this model the propagation and structure of chemical waves, corresponding to interface between bistable reactive states, and thereby determine the relative stability of these states

Island-size scaling in surface deposition processes

Diffusion-mediated nucleation and growth of islands during deposition occurs essentially irreversibly in a variety of systems. We provide a scaling theory for the full island-size distribution, both with the ratio of surface diffusion to deposition rates and with time. Scaling functions and exponents are determined by simulation and explained analytically by an unconventional rate-equation analysis. Experimental tests for theoretical predictions are discussed, including the scaling of superlattice beam profiles for diffraction studies of heteroepitaxial systems

The use of a formal sensitivity analysis on epidemic models with immune protection from maternally acquired antibodies

This paper considers the outcome of a formal sensitivity analysis on a series of epidemic model structures developed to study the population level effects of maternal antibodies. The analysis is used to compare the potential influence of maternally acquired immunity on various age and time domain observations of infection and serology, with and without seasonality. The results of the analysis indicate that time series observations are largely insensitive to variations in the average duration of this protection, and that age related empirical data are likely to be most appropriate for estimating these characteristics

Surfactant protein D contributes to ocular defense against Pseudomonas aeruginosa in a murine model of dry eye disease.

Dry eye disease can cause ocular surface inflammation that disrupts the corneal epithelial barrier. While dry eye patients are known to have an increased risk of corneal infection, it is not known whether there is a direct causal relationship between these two conditions. Here, we tested the hypothesis that experimentally-induced dry eye (EDE) increases susceptibility to corneal infection using a mouse model. In doing so, we also examined the role of surfactant protein D (SP-D), which we have previously shown is involved in corneal defense against infection. Scopolamine injections and fan-driven air were used to cause EDE in C57BL/6 or Black Swiss mice (wild-type and SP-D gene-knockout). Controls received PBS injections and were housed normally. After 5 or 10 days, otherwise uninjured corneas were inoculated with 10(9) cfu of Pseudomonas aeruginosa strain PAO1. Anesthesia was maintained for 3 h post-inoculation. Viable bacteria were quantified in ocular surface washes and corneal homogenates 6 h post-inoculation. SP-D was measured by Western immunoblot, and corneal pathology assessed from 6 h to 4 days. EDE mice showed reduced tear volumes after 5 and 10 days (each by ∼75%, p<0.001) and showed fluorescein staining (i.e. epithelial disruption). Surprisingly, there was no significant difference in corneal pathology between EDE mice and controls (∼10-14% incidence). Before bacterial inoculation, EDE mice showed elevated SP-D in ocular washes. After inoculation, fewer bacteria were recovered from ocular washes of EDE mice (<2% of controls, p = 0.0004). Furthermore, SP-D knockout mice showed a significant increase in P. aeruginosa corneal colonization under EDE conditions. Taken together, these data suggest that SP-D contributes to corneal defense against P. aeruginosa colonization and infection in EDE despite the loss of barrier function to fluorescein