ALBERT

Notizen: The vibrational quenching rate constants for NO+(v), predominantly in the v=1 state, have been measured at 200 and 293 K in collisions with H2 and D2 and at 200, 293, and 458 K with O2. The rate constants are all very low, corresponding to quenching probabilities ∼10−4. The low rate constants reflect very shallow attractive potential wells. In the case of H2 and D2 this is a consequence of their low polarizabilities. In the case of O2, repulsive chemical interactions offset the electrostatic attraction to yield a shallow attractive well. This is a consequence of the singlet NO+ and triplet ground state O2 not approaching on the attractive NO+3 ground state potential surface, which is a singlet. The temperature dependences of the quenching rate constants are generally slight, indicating that the collision energies are in a range comparable to the attractive well depth and that the quenching is not strongly dominated by either the attractive forces, which would give a negative energy dependence, or by the repulsive forces which would give a positive energy dependence.

Notizen: The rotational relaxation of the solvent Aroclor 1248 (A1248) has been examined in solutions containing either polystyrene (PS), 0≤c≤0.269 g/cc, or polybutadiene (PB), 0≤c≤0.264 g/cc, by oscillatory electric birefringence. Measurements were performed at eight temperatures from −17.31 to 25.00 °C. In all cases the normalized solvent relaxation time τ/τ0, where τ0 was the value in neat solvent, was an exponential function of concentration. For PS/A1248, ∂ ln(τ/τ0)/∂c was independent of temperature and equal to 13±1 cc/g, whereas for PB/A1248, ∂ ln(τ/τ0)/∂c increased steadily with temperature and was negative for measurement temperatures of 2.81 °C and below. These observations were qualitatively consistent with a picture of solvent clustering or ordering, which was enhanced by the addition of PS but disrupted by PB. Although some features of the data were also consistent with changes in solution free volume, as indicated by measurements of the composition-dependent glass transition temperature, such considerations were not sufficient to reconcile all the observed behavior. The ratio τ/τ0 was interpreted as representing an effective, average solvent friction function ζˆ(c,T), which could also be used to define an effective solvent viscosity, ηe(c,T)=ηs(T)⋅ζˆ(c,T). It was shown that ∂ ln ζˆ/∂c, also equivalent to an intrinsic effective viscosity [ηe], was very close in magnitude and sign to the intrinsic high frequency limiting viscosity [η'∞] for both PS/A1248 and PB/A1248. This implies that the measured values ofη∞ reflect predominantly polymer-induced changes in solvent friction. Therefore, it may not be necessary to invoke additional sources of energy dissipation, such as chain stiffness or internal viscosity, to describe the high frequency viscoelastic or oscillatory flow birefringence properties of flexible chains. Furthermore, to the extent that η'∞ reflects changes in solvent friction for any polymer/solvent system, measured values of the intrinsic viscosity [η] require reinterpretation, particularly for lower molecular weight polymers. It was also demonstrated that ζˆ(c,T), as inferred from measurements of solvent or probe diffusion in the same systems [von Meerwall, Amelar, Smeltzly, and Lodge, Macromolecules (in press)], was quite different from that reported here, indicating inter alia that the correct method for accounting for changes in local friction in polymer solutions has yet to be established.

Notizen: A two-dimensional Thomson scattering system is being designed for the Advanced Toroidal Facility (ATF) torsatron experiment. This system will provide electron temperature and density measurements at 15 points along a vertical chord with each shot from a two-dimensional Te and ne map of an ATF toroidal cross section can be obtained. The horizontal Thomson scattering viewing port is offset by 15° toriodally from the ports passing the vertical laser beam. Translating and rotating mirors will be used to relay light from the viewing port to the collection lens. This makes it possible to scan horizontally, view as much of the vertical laser beam as possible, and to use a fixed focal length, fixed position lens. Three sets of spectrometers optimized to three temperature ranges will be used. At the output of each spectrometer, dispersed light will be detected by an array of five, seven, or eight photomultiplier tubes, depending on the temperature range of the spectrometer.

Notizen: Shock-induced luminescence from lithium niobate has been studied in the stress range 1.6–21.0 GPa. Both fast-framing photography and five-channel optical pyrometry were used to observe the luminescence. The framing photography showed that the emission pattern is heterogeneous for stresses just above the dynamic yield point. A further increase of the stress resulted in a pattern which was essentially homogeneous to within the experimental spatial resolution of about 30 μm. Narrowband filters and photomultiplier tubes were used in the optical pyrometry experiments. A broadband spectrum with a peak near 700 nm was observed. A plot of the energy dissipated by the shock versus shock stress correlates very well with a plot of the 700-nm intensity versus shock stress. The mechanism for light emission in lithium niobate appears to be closely related to the dynamic yielding process.

Notizen: A quasilinear method is developed for determining saturated tearing mode magnetic island widths in axisymmetric toroidal plasmas with arbitrary cross-sectional shape, aspect ratio, or plasma pressure (beta). The method is applied to compute magnetic island widths in force-free toroidal plasmas with aspect ratio as low as 2.0 and elongation between 1.0 and 2.0. It is found that current suppression within the magnetic island strongly increases the saturated width while current peaking reduces width. The effects of current profile, geometry, and harmonic mixing are also studied.