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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. 8397-8398 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Irreversible adsorption of diatomics on crystalline surfaces is sometimes modeled as random dimer filling of adjacent pairs of sites on a lattice. We note that this process can be implemented in two distinct ways: (i) randomly pick adjacent pairs of sites, jj', and fill jj' only if both are empty (horizontal transition state); or (ii) randomly pick a single site, j, and if j and at least one neighbor are empty, then fill j and a randomly chosen empty neighbor (vertical transition state). Here it is instructive to consider processes which also include competitive random monomer filling of single sites. We find that although saturation (partial) coverages differ little between the models for pure dimer filling, there is a significant difference for comparable monomer and dimer filling rates. We present exact results for saturation coverage behavior for a linear lattice, and estimates for a square lattice. Ramifications for simple models of CO oxidation on surfaces are indicated.
    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 82 (1985), S. 2795-2810 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Models where pairs, triples, or larger (typically connected) sets of sites on a 2D lattice "fill'' irreversibly (described here as dimer, trimer, ... filling or adsorption), either randomly or cooperatively, are required to describe many surface adsorption and reaction processes. Since filling is assumed to be irreversible and immobile (species are "frozen'' once adsorbed), even the stationary, saturation state, which is nontrivial since the lattice cannot fill completely, is not in equilibrium. The kinetics and statistics of these processes are naturally described by recasting the master equations in hierarchic form for probabilities of subconfigurations of empty sites. These hierarchies are infinite for the infinite lattices considered here, but approximate solutions can be obtained by implementing truncation procedures. Those used here exploit a shielding property of suitable walls of empty sites peculiar to irreversible filling processes. Accurate results, including saturation coverage estimates, are presented for random filling of dimers, and trimers of different shapes, on various infinite 2D lattices, and for square tetramers on an infinite square lattice.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Osteoporosis international 2 (1992), S. 210-211 
    ISSN: 1433-2965
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Osteoporosis international 11 (2000), S. 158-166 
    ISSN: 1433-2965
    Keywords: Key words:Body composition – Bone density – Densitometry – Femur – Spine – Total body
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract: We assessed a new dual-energy bone densitometer, the PRODIGY, that uses a narrow-angle fan-beam (4.5°) oriented parallel to the longitudinal axis of the body (i.e., perpendicular to the usual orientation). High-resolution scans across the body can be stepped at 17 mm intervals. The energy-sensitive array detector uses cadmium zinc telluride, which allowed rapid photon counting. Spine and femur scans required 30 s, and total-body scans required 4–5 min; the dose was only 3.7 mrem and 0.04 mrem respectively, or about 5 to 10 times lower than conventional fan-beam densitometry. We found only a small influence of soft-tissue thickness on bone mineral density (BMD) results. There was also a small ( ± 1%) influence of height above the tabletop on BMD results. A software correction for object height allowed a first-order correction for the large magnification effects of position on bone mineral content (BMC) and area. Consequently, the results for BMC and area, as well as BMD, with PRODIGY corresponded closely to those obtained using the predecessor DPX densitometer, both in vitro and in vivo; there was a generally high correlation (r= 0.98–0.99) for BMD values. Spine and femur values for BMC, area and BMD averaged within 0.5% in vivo (n= 122), as did total-body BMC and BMD (n= 46). PRODIGY values for total-body lean tissue and fat also corresponded within 1% to DPX values. Regional and total-body BMD were measured with 0.5% precision in vitro and 1% precision in vivo. The new PRODIGY densitometer appears to combine the low dose and high accuracy of pencil-beam densitometry with the speed of fan-beam densitometers.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Journal of mathematical chemistry 9 (1992), S. 39-53 
    ISSN: 1572-8897
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mathematics
    Notes: Abstract We present the exact solution for the sequential, random, irreversible filling of one-dimensional lattices by linear n-mers using the end-on filling mechanism. The results are extrapolated to then → ∞ limit (a variation on the car-parking problem) to yield a saturation coverage (packing density) of 0.7350. The end-on filling mechanism involves two steps for a single filling event. First, the landing site for one endpoint of the filling species is chosen and then the second endpoint is subsequently chosen (fromunfilled sites an appropriate distance from the first endpoint). We compare this mechanism to the conventional, one-step filling mechanism, where both endpoints of the filling species are chosen simultaneously. We present results detailing how the lattice saturation coverage varies for the two mechanisms. In addition, we extend our analysis to consider filling in the presence of a time-dependent, random distribution of inactive sites.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Journal of statistical physics 69 (1992), S. 151-162 
    ISSN: 1572-9613
    Keywords: Random and cooperative sequential adsorption ; jamming coverage ; ladders ; strips ; shielding
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We analyze various processes where particles are added irreversibly and sequentially at the sites of infinite ladders or broader strips (i.e., on terraces) of adsorption sites. For “sufficiently narrow” strips or ladders, exact solution in closed form is possible for a variety of processes. Often this is most naturally achieved by mapping the process onto an equivalent one-dimensional process typically involvingcompetitive adsorption. We demonstrate this procedure for sequential adsorption with nearest-neighbor exclusion on a 2×∞ square ladder. For other select processes on strips “slightly too broad” for exact solution, “almost exact” analysis is possible exploiting an empty-site shielding property. In this way, we determine a jamming coverage of 0.91556671 for random sequential adsorption of dimers on a 2×∞ square ladder. For “broader” strips, we note that the complexity of these problems quickly approaches that for ∞×∞ lattices.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    Journal of statistical physics 38 (1985), S. 681-705 
    ISSN: 1572-9613
    Keywords: Dimer filling ; lattice ; irreversible ; saturation ; hierarchy equations
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The recombination of nearest neighbors in a condensed matrix of free radicals was modeled by Jackson and Montroll as irreversible, sequential, random dimer filling of nearest-neighbor sites on an infinite, three-dimensional lattice. Here we analyze the master equations for random dimer filling recast as an infinite hierarchy of rate equations for subconfiguration probabilities using techniques involving truncation, formal density expansions (coupled with resummation), and spectral theory. A detailed analysis for the cubic lattice case produces, e.g., estimates for the fraction of isolated empty sites (i.e., free radicals) at saturation. We also consider the effect of a stochastically specified distribution of nonadsorptive sites (i.e., inert dilutents).
    Type of Medium: Electronic Resource
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