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  • American Institute of Physics (AIP)  (9)
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  • 1
    Electronic Resource
    Electronic Resource
    Estimation of homogeneous nucleation flux via a kinetic model (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 8302-8309 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The proposed kinetic model for condensation under homogeneous conditions, and the onset of unidirectional cluster growth in supersaturated gases, does not suffer from the conceptual flaws that characterize classical nucleation theory. When a full set of simultaneous rate equations is solved, a characteristic time emerges, τn, for each cluster size, at which the production rate, and its rate of conversion to the next size (n+1) are equal. Procedures for estimating the essential parameters are proposed; condensation fluxes Jsskin are evaluated. Since there are practical limits to the cluster size that can be incorporated in the set of simultaneous first-order differential equations, a code was developed for computing an approximate Jssth, based on estimates of a "constrained equilibrium'' distribution, Nequ, and identification of its minimum, Neql.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460115
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  • 2
    Electronic Resource
    Electronic Resource
    Metastable size distributions of molecular clusters in supersaturated vapors (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 9408-9412 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The shapes of size distributions of molecular clusters in highly supersaturated vapors, assuming they remain unrelaxed, i.e., in "constrained equilibrium," were derived for three representative systems. The required thermochemical parameters were estimated via the kinetic molecular model that accurately reproduced the measured condensation fluxes over a wide range of temperatures and supersaturation levels. The computed size distributions are neither log–normal nor follow an exponential-type decline with increasing cluster size. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1370076
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  • 3
    Electronic Resource
    Electronic Resource
    Thermochemical and kinetic parameters for hydrogen bonded clusters, derived from avalanche condensation flux measurements (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 10192-10196 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The kinetic molecular model (KMM) was applied to recently reported extensive measurements of condensation flux levels developed under homogeneous conditions from supersaturated vapors of water and n-pentanol, in inert gas carriers. These data span wide ranges in temperatures and critical supersaturations (css). The calculated fluxes reproduced those observed (defined: number of droplets generated per cm3-s) within a factor 2, utilizing a narrow range of adjustable parameters. From the derived kinetic and thermochemical parameters one may derive estimates of size-dependent entropies for clusters. Thus, the derived entropy per mole of a 13 unit cluster of water molecules, at 233 K, is 233 eu. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481660
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  • 4
    Electronic Resource
    Electronic Resource
    Spectral signatures of critical transition structures derived from temperature-dependent IR spectra (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 5904-5910 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We describe a modified configuration of the temperature drift (TD) technique for recording infrared spectra that arise from vibrationally excited states. TD is a differential measurement that records decrements in IR absorptions due to modest increases in temperature. In particular, it permits identification of oscillators that are coupled to large amplitude, low frequency motions associated with transition structures, such as transitions to free rotations about single bonds that are hindered at ambient temperatures. Selected portions of the spectra of two species were recorded to demonstrate the feasibility of locating spectral features of critical transition structures. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470470
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  • 5
    Electronic Resource
    Electronic Resource
    Kinetics of the condensation of Cs from its supersaturated vapor: Comments on the entropy of a small Cs cluster (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 8265-8266 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The recently published measurements of temperature-dependent critical supersaturation levels for avalanche condensations of Cs vapor are interpreted via the recently revised kinetic molecular model that incorporates accretions with dimers as well as monomers. The kinetic parameters required to reproduce the reported condensation rate of, approximately, unit cluster per cm3 s permits estimations of the entropies of clusters. Therefrom the mean intermolecular (bulk) frequency derived for Cs13 (466 K) fits well with a model proposed by Hoare for Xe13. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480333
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  • 6
    Electronic Resource
    Electronic Resource
    Kinetics of condensation in supersonic expansion (Ar) (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 2029-2037 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The temperature and density in an overexpanded jet (Ar) were mapped by the method of characteristics. The kinetics of condensation in the supersaturated gas along the central streamline was analyzed on the basis of four models, subject to previously developed thermodynamic relations which must apply during the homogeneous condensation of any supersaturated vapor. Here we report on detailed dynamic analyses of the rates of cluster growth. Each association: An−1+A1(arrow-right-and-left)An was represented either by a single stabilization step, or by a sequence of two energy removing collisions, for which kinetic parameters based on Lennard-Jones (LJ) potentials were estimated. Coupled differential rate equations (up to 192) were solved, both for cases when the heat of condensation was neglected, and when it was partially or fully distributed throughout the fluid. The distribution of cluster sizes declines exponentially, according to N(n)=A exp(−an)+B exp(−bn), in agreement with generally observed distributions for monatomic species. For source pressures of 3 and 10 atm the condensation process becomes kinetically controlled within a distance downstream somewhat less than twice the aperture diameter. Our calculations indicated that after about three aperture lengths "freezing in'' takes place if the heat of condensation is neglected. When the heat of condensation is distributed (partitioned in one of several plausible ways) freezing in is delayed beyond four aperture diameters. A modest fraction of the nascent clusters retain some of the heat of condensation and remain at a higher temperature than the ambient gas.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451147
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  • 7
    Electronic Resource
    Electronic Resource
    Kinetic mechanism for condensation from supersaturated vapors that incorporate high levels of dimers (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 7926-7934 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The kinetic molecular mechanism (KMM) for the onset of avalanche condensation from supersaturated vapors, described in this report, is more general than the one we used previously. While the expressions for the rate constants needed to compute the steady-state populations are the same, additional steps were introduced to take into account cluster growth via accretions with dimers. In the present version, the magnitudes for the stepwise enthalpy decrements are assigned using all the "best" available experimental and computed values, so that the program requires less arbitrary adjustable parameters. The derived values for the entropy decrements upon successive accretions present a basis for estimating the total entropy of clusters (size n), and thus impose constraints on any model for dynamics within clusters. For high ambient pressures an option is provided for correcting the condensation rate constants due to restricted diffusion of accreting units (monomers or dimers) into the reaction shell of the growing clusters. Published data for condensation flux levels in supersaturated methanol are reanalyzed to illustrate the use of the newly developed code. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478725
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  • 8
    Electronic Resource
    Electronic Resource
    The dissociation of dimeric aliphatic acids: Energetics and relaxation kinetics (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 5794-5803 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: After a brief listing of the available structural, spectroscopic, and thermochemical data for gaseous formic acid, we summarize the results of quantum mechanical computations of minimum energy structures and standard entropies for the ring and chain dimers and for the monomer. The computed increments in thermochemical parameters for interconversions between these species are compared with values derived from measured variations of equilibrium constants with temperature. Rate constants were estimated both for the limiting high and low pressure regimes; these relaxation times were computed for several possible mechanisms. At low pressures, the computed activation energy for one of several sequential processes is consistent with that derived from the temperature- dependet relaxation times. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474339
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  • 9
    Electronic Resource
    Electronic Resource
    The kinetic-molecular model for condensation flux magnitudes applied to binary mixtures (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 4651-4655 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The kinetic-molecular model (KKM) for calculating the rates of condensation of supersaturated unary vapors has been extended to ideal and nonideal binary vapors by treating such mixtures as pseudovapors with properties assigned according to a linear combination of the properties for the pure components in the vapor phase. The single empirical KMM parameter, the hot-cluster stabilization parameter (SP) was found to vary with vapor composition, with deviations from linearity that could be correlated with cluster compactness; indeed, SP provides a measure of this structural feature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466063
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