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  • 1995-1999  (3)
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Material
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  • 1
    Electronic Resource
    Electronic Resource
    Supercooling of H2 clusters produced in free-jet expansions from supercritical states (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 6258-6271 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A combined theoretical and experimental investigation has been undertaken to determine optimum conditions for achieving rapid cooling of H2 clusters in nozzle-beam expansions with the goal of producing superfluid H2 clusters. Theory predicts that a temperature less than 6.6 K, well below the 13.8 K triple-point temperature of p-H2, is required. Terminal specific enthalpies of clusters are determined experimentally from terminal velocities of clusters measured using the time-of-flight technique. The results are interpreted in the context of isothermal and adiabatic spinodals for p-H2 constructed using thermodynamic methods and a van der Waals equation-of-state model. The lowest terminal enthalpies are achieved in expansions starting from supercritical source conditions and crossing the binodal curve of the phase diagram with densities far to the liquid side of the critical point. In this case the clusters are formed via relatively late fragmentation of metastable liquid H2. These clusters are expected to be liquid and, at the point in the expansion at which collisions cease, to have temperatures of about 9 K. Before arriving at the detector they are cooled further by evaporation to temperatures between 4 and 5 K. Further experiments are needed to determine if these clusters are superfluid. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469072
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Average size and size distribution of large droplets produced in a free-jet expansion of a liquid (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 2664-2668 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The experimental parameters and fluid properties affecting the average size N¯ and the size distribution P(N) of droplets formed by fragmentation of a liquid after expansion into a vacuum are investigated. The mean droplet size is found to be a function of the surface tension of the liquid, the nozzle diameter, and a characteristic flow speed. The size distribution is found to be a linear exponential distribution; measurements deviate from this distribution at small sizes if a factor which is a function of the cluster size is included in the measuring process. Good agreement with measured distributions of both positive and negative droplet ions formed from neutral 4He droplets by electron impact is found. The strong dependence of mean droplet size on source–orifice diameter found in the present analysis indicates that earlier correlations of droplet size with specific entropy in the source were useful at best only for a fixed nozzle size. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477988
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Size correlations for condensation clusters produced in free-jet expansions (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 9125-9132 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: For clusters produced by condensation from a gas phase, a basis for identifying the dimensionless thermodynamic and kinetic parameters which characterize cluster formation is provided by arranging an expression for the mass rate of production of critical-size clusters into dimensionless form. The experimentally observed dependence of the cluster size on source pressure dictates the choice between two alternative classical expressions for the nucleation rate. An earlier analysis of dimer formation in free jets guides the combining of the thermodynamic and kinetic parameters into a single scaling parameter which facilitates correlating the size of clusters in free jets for all rare gases, including 4He, using a single curve. Important facets of the correlation are (a) the explicit dependence of the scaling parameter on the surface tension and, for the rare gases other than 4He, the evaluation of the surface tension at the triple point, and (b) the use of real-fluid properties for the density and sound speed of the 4He in the source. An earlier conclusion that size distributions for large condensation clusters can be correlated with dimer partial pressure in the source is found to be fortuitous. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475204
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    Paper (German National Licenses)
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