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  • Springer  (4)
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  • 1
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A molecular beam apparatus of high angular resolution is described, and measurements of the angular intensity distribution of alkali beams scattered at small angles (15 seconds of arc to 1 degree) by crossed beams of various gases and vapours are presented. Formulas, connecting the measured intensity with the differential cross section in the center of mass system, are derived, considering the velocity distribution in the beams and the geometry of the experimental set-up. The measurements show, that the differential cross section can be calculated down to very low angles (order of magnitude 10 minutes of arc) by means of classical mechanics, taking only into account the long-range attractive van der Walls interaction. At smaller angles, deviations, predicted by quantum mechanics, can be observed. If the potential of the van der Waals forces is written in the formV(r)=−C/r s, a method is outlined to determine bothC ands from the measured angular intensity distribution.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Total cross sections for collisions of Helium atoms with other rare gases have been measured by modulated molecular beam technique. The results are used to determine the interaction constants of the van der Waals potential (V(r)∼ −C/r 6). For this purpose, an indirect procedure is adopted, since the well known approximation ofMassey andMohr (Q∼C 2/5) is not applicable for Helium scattering experiments at thermal energies. The van der Waals constants obtained are in agreement with values determined from experimental data on transport properties in gases.
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  • 3
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The velocity dependence of the total elastic cross section has been measured for the scattering of H2, D2, and He beams by Ne, Ar, Kr, and Xe gases. Velocity-selected beams were attenuated by gas in a scattering chamber. A universal detector, with a magnetic mass separator, was used in conjunction with lock-in techniques. The results are compared with quantum mechanical calculations using a Lennard-Jones-(12.6)-potential, and employing suitable averaging over experimental velocity distributions. Potential parameters are presented. These are in good agreement with those expected from other methods of measurement.
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  • 4
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Scattering experiments are often not performed with the ideal set-up consisting of a monochromatic beam and target particles at rest. This is, for instance, never the case in molecular beam scattering, which is of primary interest in this paper. Two arrangements are normally employed, one consisting of a gas chamber as target (case a), the other of a crossed beam, generally at right angles to the incoming beam (case b). In both cases the scattering particles have a Maxwellian velocity distribution. This means that in any such experiment an “effective cross section” is measured, which is an integral over cross sections for different collision velocities. Other similar integrals are needed, if one works with polarized beams and takes into account the fact that the cross section is angular dependent. A third type of integral is required, when the primary beam is not monochromatized, and one has to average over its velocity distribution too. The functions needed to interpret scattering experiments of all the types mentioned are given in this paper, as well as a table of values for the 24 most important of these functions.
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