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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 1019-1029 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The experimentally measured rotational diffusion times of various neutral solute molecules are compared with hydrodynamic and molecular models for microscopic friction. New viscosity and temperature dependent results for the rotational time of a large neutral solute (26 A(ring) long) indicate nearly perfect agreement with stick hydrodynamic predictions in both alkane and alcohol solvents. For smaller solute molecules, the results of previous studies show an increasingly large deviation from hydrodynamic predictions as the solute size decreases. This solute size dependent effect is compared with the theoretical predictions of Zwanzig's hydrodynamic "bumpy cylinder'' model, Dote–Kivelson-Schwartz's molecular free volume model, and a new generalized hydrodynamic model for the effects of frequency dependent viscosity on molecular rotational diffusion. Although hydrodynamic predictions agree with experiment in some respects, it is found that molecular aspects of the solvation process must be introduced in order to get quantitative agreement with the observed solute size dependence.
    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 89 (1988), S. 5224-5236 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The decay of an initially excited donor due to the presence of acceptor molecules embedded in porous silica gels has been studied. The time evolution of the decay process has been related to the geometrical restrictions imposed on the participating molecules by the porous structure of the silicas. Fractal and regular geometrical shape models have been confronted with experimental findings. We conclude that for silica gels simple pore geometries, i.e., spheres and cylinders, account well for the relaxation behavior. The decay patterns exhibit temporal crossovers typical of dynamics in restricted geometries.
    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 89 (1988), S. 7287-7297 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The vibrational circular dichroism (VCD) of dimethylcyclopropane is studied by ab initio techniques. The equilibrium geometry, the molecular force field, and the matrix elements for the infrared (IR) and VCD intensities are evaluated using the derivative method for molecular wave functions of SCF type. An alternative derivation of Stephens' expression for the rotational strength is presented. The results are compared with the available experimental information for the C–H stretching modes. This is an excellent demonstration of the validity of this theory for the VCD intensity of the C–H vibrations, which are easily rationalized using group theoretical arguments.
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 57 (1986), S. 2101-2106 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The goals of the recently activated Nova laser facility are to address critical issues for evaluating the feasibility of inertial confinement fusion, to implode DT to densities exceeding 200 g/cm3 and pressures greater than 1011 atm, and to perform a wide range of high energy density plasma physics experiments in the areas of XUV/x-ray lasers, hydrodynamics, and radiation generation and transport. An extremely flexible and sophisticated facility is required to successfully perform such a variety of tasks. The ten-arm Nova laser is capable of irradiating complex targets with laser wavelengths of 0.53 and 0.35 μm and pulse widths that range from 0.09 to 〉5 ns, and peak powers greater than several terrawatts per beam line. A sophisticated, variable impedance, transmission line driven Pockels cell allows for complex temporal shaping of the laser pulse. Synchronized oscillators allow for different pulses to be propagated down the beam lines for experiments that require long-pulse or short-pulse x-ray backlighting. The output of the laser can be directed into two independent target areas: a 4.4-m-diam vacuum vessel for experiments which require 10 beams, and a 1.8-m-diam chamber for two Nova arms. The facility has been designed to allow nearly simultaneous, independent experiments to be conducted in both target areas. A number of sophisticated optical, XUV, x-ray, and particle diagnostics measure target performance. An optical fiducial system allows cross correlation of all of the diagnostic systems to better than 50 ps. An overview of the facility, diagnostics, and data-acquisition system will be discussed.
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 56 (1985), S. 844-844 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We have developed a streaked crystal spectrograph for time-resolving x-ray lines from laser-produced plasmas. The spectrograph combines an ellipsoidally curved crystal and x-ray streak camera to optimize spectral coverage and resolution, even though the instrument is located a meter from the source. The instrument has been used at Novette to measure S lines doped in concentrations of 4% in the center of CH foils. Time histories of the temperature and density of the CH plasma have been derived from relative intensities of the S lines. Details of the experiment and analysis will be presented along with the instrument design.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 1673-1678 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the results of light scattering, absorption, excitation, and emission spectroscopy of three polyphenylene vinylene (PPV) derivatives; poly[2-methoxy, 5-(2'-ethyl-hexyloxy-p-phenylene- vinylene] (MEH-PPV), poly[2-butoxy, 5-(2'-ethyl-hexyloxy-p-phenylene-vinylene] (BEH-PPV), and poly[2-dicholestanoxy-p-phenylene-vinylene] (BCHA-PPV) in solution with p-xylene. We find that increasing the size of the solubilizing side chains increases the intrinsic persistence length of the polyphenylene vinylene backbone and that this change in stiffness has dramatic effects on the photoluminescence of polyphenylene vinylene. We have determined the luminescence quantum efficiencies of the polyphenylene vinylene derivatives relative to a known standard, Rhodamine 6G, and find that the photoluminescence can be greatly enhanced by increasing the intrinsic stiffness of the polymer backbone. The stiffest polymer, poly[2-dicholestanoxy-p-phenylene-vinylene] (BCHA-PPV), has a quantum efficiency of 0.66±0.05. The quantum efficiency decreases to 0.22±0.05 for poly[2-butoxy, 5-(2'-ethyl-hexyloxy-p-phenylene-vinylene] (BEH-PPV) and 0.20±0.05 for poly[2-methoxy, 5-(2'-ethyl-hexyloxy-p-phenylene-vinylene] (MEH-PPV), the most coiled derivative. Excitation profiles of the three derivatives also show an increase in nonradiative decay at high energies when the polymer assumes a more coiled comformation. Thus, the quantum yields are dependent on pump energy.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 8089-8095 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report "exact'' quantum Monte Carlo calculations of the potential energy surface for the reaction H+H2→H2+H. The method used is free of systematic error. The statistical or sampling error was reduced to ±0.10 kcal/mol for several hundred points distributed across the surface, to ±0.02 kcal/mol for the minimum energy approach of H to H2, to ±0.02 kcal/mol near the saddle point, and to ±0.01 kcal/mol at the saddle point. The upper and lower surfaces in the region of the Jahn–Teller cusp were determined with a statistical error of ±0.2 kcal/mol.
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  • 8
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 4690-4698 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Naturally occurring shocks transport energy and accelerate particles throughout the cosmos. The problem of producing collisionless shocks in the laboratory that are of relevance to such cosmic shocks is considered. Such an experiment must meet a number of constraints, several of which can be expressed by algebraic scaling relations. The relations for magnetization, plasma beta, Alfvén Mach number, temperature, magnetic field, and collisionality are described here. Taken together, the limits imposed by these constraints upon possible experiments are specified. The growth of magnetohydrodynamic (MHD) turbulence and the degree of particle acceleration are examined, demonstrating that it is feasible to contemplate studies of such phenomena in the laboratory. Finally, some discussion of how an experiment might meet the other qualitative constraints, and of how a laser might be used to drive the shock, is also included. © 2000 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 2797-2801 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The stability of the tokamak edge pedestal to ballooning modes is addressed using three-dimensional simulations of the Braginskii equations and simple analytic models. The effects of ion diamagnetic drift and the finite radial localization of the pedestal pressure gradient are found to be strongly stabilizing when δ〈δR, where δ is the pedestal half-width and δR∼ρi2/3R1/3 in the center of the pedestal. In this limit, conventional ballooning modes within the pedestal region become stable, and a stability condition is obtained in the two fluid system α/αc〈(4/3)δR/δ (stable) which is much less stringent than that predicted by local magnetohydrodynamic (MHD) theory (α/αc〈1). Given α∼q2Rβ/δ, this condition implies a stability limit on the pedestal β: β〈βc, where βc=(4αc/3q2)δR/R. This limit is due the onset of an ideal pressure driven "surface" instability that depends only on the pressure drop across the pedestal. Near marginal conditions, this mode has a poloidal wavenumber kθ∼1/δR, a radial envelope ∼δR(〉δ), and real frequency ω∼cs/δRR. © 1999 American Institute of Physics.
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  • 10
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 59 (1988), S. 1801-1803 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Experiments to obtain time-resolved, soft-x-ray emission from laser-driven plasmas are succinctly described. The spectra (0.19 keV ≤ hν ≤ 1.3 keV) at various times have been deconvolved and energy integrated to obtain time-resolved yields from Au disk targets. The temporal profiles of the total thermal x-ray output power follow the overall laser temporal shape but do not show the high-frequency fluctuation observed in the laser pulse. The behavior of the ratio of the instantaneous x-ray yields over the laser absorption is studied. The studies were done at the LLNL Nova laser facility. Single pulses and pulses in "picket fence'' configuration were used to heat the gold targets.
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