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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 11505-11510 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The homogeneous nucleation of supersaturated cesium vapor was investigated in a thermal diffusion cloud chamber operating in both the upward and the downward mode. In the upward operating mode, critical supersaturations were measured in the temperature range, 446–492 K. By operating the chamber in the downward mode, it was possible to circumvent experimental difficulties which arise at low pressures (due to phoretic effects) for the measurements made in the upward mode. Our previously published measurements on cesium were obtained in the upward mode in the temperature range, 421–554 K. These earlier measurements agree perfectly (to within their scatter) with the measurements presented here in the temperature range where they overlap, i.e., 421–492 K. The use of the downward mode enabled the extension of the temperature range of the measurements by 132 K towards lower temperatures. This makes cesium the substance whose homogeneous nucleation has been measured, in a thermal diffusion cloud chamber, over the largest range of temperatures, i.e., 289–554 K. The measured critical supersaturations were compared to the predictions of the Internally Consistent version of Classical Nucleation Theory and to the predictions of the Classical Theory. © 1999 American Institute of Physics.
    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 112 (2000), S. 8363-8366 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper describes the development of a novel detection method and the demonstration of its capability to detect substances at concentrations as small as a few parts per trillion. It is shown that photoinduced nucleation is not in itself a nucleation process; rather, supersaturated vapor condenses onto long-lasting clusters formed by chemical reaction of photo-excited molecules. The role of the supersaturated vapor is to increase the size of these photoproducts by condensation to a size readily detectable by light scattering. Furthermore, the measured nucleation rate variation with illumination wavelength exactly matches the substance's vapor-phase UV light absorption wavelength dependence, thus providing species identification. The ability to detect and identify molecules of substances at extremely low concentrations from ambient air is useful for detecting and monitoring pollutants, and for detecting explosives such as TNT. © 2000 American Institute of Physics.
    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 111 (1999), S. 3623-3629 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Homogeneous nucleation rates of n-pentanol as functions of both supersaturation and temperature were measured in two different upward thermal diffusion cloud chambers, by research groups in Prague and Baltimore. The measurements were made at temperatures between 280 K and 320 K. The nucleation rates obtained are compared to the rates measured by Luijten et al. [J. Chem. Phys. 106, 4152 (1997)], by Hrubý et al. [J. Chem. Phys. 104, 5181 (1996)], and by Strey et al. [J. Chem. Phys. 84, 2325 (1986)]. Fair agreement between our data and the data obtained by the other authors also was found. © 1999 American Institute of Physics.
    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 101 (1994), S. 2309-2318 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new method for measuring steady-state nucleation rates on ions in an upward thermal diffusion cloud chamber is described. It provides quantitative measurement of all relevant variables, i.e., temperature, supersaturation, and ion density. Each of these variables, as well as the ion sign and the chemical makeup of the ions, can be readily changed and investigated. Using this technique, it is shown that the nucleation rate is directly proportional to the ion density. It is also shown that there is no dependence of the nucleation rate on electric field strength nor is there a dependence on the ion's chemical makeup.
    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 105 (1996), S. 4707-4713 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Homogeneous nucleation rates as functions of both supersaturation and temperature were measured in an upward thermal diffusion cloud chamber for four n-alkanes: n-heptane, n-octane, n-nonane, and n-decane. Nucleation rates from about 10−4 to 5×100 drops cm−3 s−1 were obtained in the temperature range, 241 to 330 K. Their dependences on supersaturation and temperature were compared to predictions of several nucleation theories: the internally consistent Classical theory, two versions of the Kalikmanov–van Dongen theory, and the Delale–Meier theory. Each theory predicted the dependence of the nucleation rate on supersaturation reasonably well. However, large temperature dependent correction factors were needed for quantitative agreement between measured and predicted nucleation rates. A plot of the ratio of measured to predicted nucleation rates vs reduced temperature shows that all n-alkanes investigated can be represented to within a factor of ten by a single, best fit line. © 1996 American Institute of Physics.
    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 104 (1996), S. 8649-8656 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Calculations of temperatures and supersaturations in nucleation experiments often include the assumption that all gases are ideal and they ignore the increase in vapor pressure due to the presence of noncondensible gases. Regardless of experimental technique, such assumptions can lead to substantial errors when comparing experiment to theory, especially when included in only one or the other. To demonstrate these effects, the procedures for calculating the temperatures and supersaturations in thermal diffusion cloud chambers are examined in detail. Nonideal gas effects do alter them; however, these effects also alter the rates calculated by nucleation theory and the effect is larger on nucleation theory than on the analysis of the experimental data. Due to the presence of noncondensible gases there also is a Poynting type effect, but it very nearly cancels in thermal diffusion cloud chambers when included consistently in both theory and experiment. An additional effect which arises only in nucleation theory is the nonzero compressibility of the critical nuclei. The net result (for n-nonane), when these effects are included consistently in both experiment and theory, is to reduce the temperature dependence of the multiplicative correction to theory necessary for agreement with experiment. None of these effects account for the previously observed apparent decrease in nucleation rate with increasing total pressure. Also critically examined are the uncertainties in the transport properties used to calculate the chamber profiles. Of particular interest is the uncertainty in the thermal diffusion factor which changes systematically with composition and thus with total pressure. © 1996 American Institute of Physics.
    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 104 (1996), S. 8657-8661 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A key approximation in the analysis of upward thermal diffusion cloud chambers is that both the heat flux and mass flux (in the center part of the chamber) are plane parallel. Procedures for determining when this approximation is valid are described. The key experimental parameter is the amount of heat added to the chamber walls to prevent vapor from condensing on them. Underheating the walls causes the experimental rates to be as much as 105 smaller than they would be when the plane parallel flux approximation is valid. Overheating the wall causes convection in the chamber center, also invalidating the plane parallel flux approximation. However, there does exist a range of wall heat values between these under and overheating limits over which the nucleation rate is negligibly affected. The overheating limit depends on the type and amount of noncondensible carrier gas present, whereas the underheating limit does not. Decreasing the molecular weight of the noncondensible gases, or decreasing the noncondensible gas pressure, increases the overheating limit. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 459-467 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Initial measurements of the homogeneous nucleation of supersaturated cesium vapors using an upward thermal diffusion cloud chamber specially designed for use with highly reactive materials are presented. Comparison of the supersaturations measured at higher temperatures (greater than 455 K) with the supersaturations predicted by Classical nucleation theory show for the first time that the nucleation behavior of simple normal metals can be described by Classical theory. This agreement implies that the description of relatively large clusters of simple metal atoms (more than 25 atoms) as small liquid droplets with bulk metallic properties is valid. However, as the temperature decreases below 455 K, an increasingly larger supersaturation than that predicted by theory is required. This deviation from theory is so sharp that by 430 K the difference is an order of magnitude, and below 420 K nucleation cannot be observed. The deviation is discussed in the context of loss of bulk metallic properties with decreasing critical nuclei size. Other possible reasons for a deviation towards larger supersaturations are also considered.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Journal of regional science 21 (1981), S. 0 
    ISSN: 1467-9787
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geography , Economics
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Journal of regional science 22 (1982), S. 0 
    ISSN: 1467-9787
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geography , Economics
    Type of Medium: Electronic Resource
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