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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 6875-6889 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Liquid helium clusters are produced by expanding gaseous 4 He into a vacuum from a cold source with temperatures between 5 and 20 K at stagnation pressures from P0 =8 to 20 bar and are studied by time-of-flight (TOF) and mass spectrometry. At low temperatures, T0 〈12 K, the mass spectra show several anomalies which can be attributed to pick-up of residual gases. At T0 〈10K, there is evidence for a very intense peak at m=16 amu which is attributed to He+4 . Depending on the temperatures, the TOF spectra reveal ions with three different velocities. These TOF observations are analyzed using isentropic lines in the known phase diagram of 4 He, which take into account deviations from ideal gas behavior. Three qualitatively different expansion regimes are identified: (I) the expansion proceeds through a region on the high temperature side of the critical point, (II) the expansion passes through or near the critical point, and (III) the expansion passes through a region on the low temperature side of the critical point. The mass spectra, peak velocities and speed ratios, when analyzed with the aid of the phase diagram, indicate that (a) two of the TOF peaks are due to clusters, (b) the fastest cluster peak is due to clusters formed by condensation of gas phase atoms, and (c) the slowest cluster peak is due to either separation into two phases (regime II) or disintegration of a liquid phase (regime III). Measured conversions of initial enthalpy into free jet kinetic energy suggest that the cluster temperature undergoes a sharp drop to a very low temperature approaching 0 K at T0 〈6.5 K where the expansion isentrope intersects the liquid–vapor line upstream from the source orifice.
    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 85 (1986), S. 2653-2663 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An HF nozzle beam and a cw HF laser beam in a coaxial and antiparallel arrangement facilitated exciting HF molecules vibrationally in v=1,J up to the saturation limit. A beam consisting exclusively of monomeric HF was generated by heating the beam source to 600 K and was confirmed using a mass spectrometer. Energy-flux measurements made using a bolometer indicate that the vibrational energy distribution initiated by the laser is conserved, confirming that V–T energy transfer to monomers is slow. The maximum measured flux of HF(v=1,J) molecules was 1.5×1017 molecules s−1 sr−1 at 670 mbar stagnation pressure. Beams consisting of a mixture of monomeric and polymeric HF were generated using appropriate combinations of source temperature and pressure. In the presence of dimers, the excited monomers relax rapidly by a fast V–V transfer and subsequent predissociation of the metastable dimers. The loss of beam energy due to energy transport by the departing dimer fragments was detected bolometrically. The laser-induced depletion of dimers was determined over a wide source pressure range at T0=300 K using a mass spectrometer. From a kinetic model of these dimer-depletion data, the product of cross section and transfer probability for V–V energy transfer from HF (v=1) monomers to dimers was deduced to be 14 A(ring)2. The terminal-dimer-concentration equation for HF nozzle beams was refined. An analysis of the energy fluxes measured by the bolometer with no laser excitation of the beam revealed that, for an HF nozzle beam, the flow from the source to the detector is, in general, nonisenthalpic due to the onset of dimerization. Mach-number focusing enriches the dimer mole fractions detected by the bolometer. An analysis of the excess enthalpy due to the presence of polymers in the beam results in a new method for the experimental determination of the dissociation energy of HF dimers, determined to be 0.36 eV.
    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 102 (1995), S. 6258-6271 
    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
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 60 (1989), S. 344-346 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The nonresonant heating of a supersonic beam nozzle induced by antiparallel coaxial irradiation by a cw Ar+ laser has been investigated time dependent using a mass spectrometer and an Ar beam. The resulting laser-induced decrease of beam density as a function of laser power and source pressure is quantitatively modeled, leading to an accurate measure of effective nozzle temperature and beam heating.
    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 110 (1999), S. 2664-2668 
    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
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 9125-9132 
    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
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  • 7
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 88 (1984), S. 4513-4521 
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Industrial and engineering chemistry 3 (1964), S. 55-60 
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Type of Medium: Electronic Resource
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